More Than 101 Ways to Use a Laser
Edited by: Herb Gottlieb
The availability of low-power helium-neon lasers and visible diode lasers makes it possible to demonstrate many of the concepts of science and technology that had been impossible or impractical in the past.
It is extremely difficult to identify all of the scientists and teachers who were the first to originate the demonstrations or applications cited here. If any originator has been omitted please let us know so we can give full credit in the future.
Lasers manufactured by Metrologic, as well as those made by other manufacturers, can be used in the ways that are presented here. However, all of the Metrologic lasers and accessories have been engineered and class tested to work together in the educational environment where rapid setup and reliable performance are essential.
If more details are needed concerning the scientific concepts presented here, we suggest that you refer to current physics textbooks and the Metrologic publications listed in this website.
Metrologic Instruments offers this information in the transparent hope that by helping you, the teacher, you will in turn help us.
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1. Scattering, Beam Visibility in Air
2. Scattering, Crystal Imperfection
3. Scattering, Rayleigh Effect
4. Scattering, Size of Particles
5. Scattering, Monitoring Fluid Purity
6. Color, Monochromaticity of Laser Light
7. Color, Absorption of White Light
8. Color, Absorption of Laser Light
9. Color, Quantum Energy of Red Light
10. Color, Coefficient of Absorption
11. Color, Reflection from Opaque Surfaces
12. Reflection, Specular Versus Diffuse
13. Reflection, Internal Reflection
14. Reflection, Law of Reflection
16. Reflection, The Colors of the Rainbow
17. Reflection, Critical Angle
18. Internal Reflections in a Water Jet
19. Critical Angle, Minimum Radius of Curvature of Light Pipes
20. Index of Refraction, Liquids
21. Index of Refraction, Liquid with Varying Optical Density
22. Index of Refraction, Glass
23. Index of Refraction, Prism
24. Variable Index of Refraction, Schlieren Effect
26. Laser Characteristics, Beam Diameter
27. Laser Characteristics, Beam Divergence
28. Laser Characteristics, Polarization
32. Geometric Optics, Lens Focal Length
34. Beam Shaping, Magnification
35. Beam Shaping, Lens Focal Lengths
37. Beam Shaping, Depth of Field and Beam Waist Diameter
40. Diffraction, Variable Slit
42. Diffraction, Multiple Slit
44. Diffraction, Reflections from a Ruler
46. Interference, Multiple Internal Reflections in Glass
47. Interference, Window Glass
48. Interference, Heat Expansion
49. Interference, Evaporation of Alcohol
50. Interference, Michelson Interferometer
51. Interference, Changes in Volume
52. Interference, Lloyd's Mirror
53. Modulation, Morse Code Communication
54. Modulation, Voice Communication
55. Modulation, Video Transmission
56. Speed of Light, Modulation Method
57. Holography, Observing Holograms
58. Holography, Making White Light Reflection Holograms
59. Holography, Cylindrical Holograms
60. Holography, Transmission Holograms
61. Hologram, Multiple Channel
62. Holography, Holographic Interferometry
64. Applications, Optical Lever
65. Applications, Optical Galvanometer
66. Applications, Pulse Indicator
68. Applications, Inspecting Wobble or Height
69. Applications, Measurement, Wire Diameter
70. Applications, Foucault Knife Edge Test
71. Applications, Spatial Filter
72. Applications, Rangefinding
73. Applications, Wheel Alignment
74. Applications, Laser Oscilloscope
75. Applications, Detect Flaws
76. Applications, Lissajous Patterns
78. Applications, Measurement by Scanning
79. Applications, Ophthalmology
80. Application, Measure Blood Cell Diameter
81. Applications, Doppler Effect
82. Applications, Calibrating a Diffraction Grating
83. Applications, Measuring Wavelength of a VLD Laser
84. Applications, Leveling Optical Benches
85. Measuring the Speed of Light, Mechanical
86. Physical Optics, Fraunhofer Diffraction
87. Physical Optics, Filtering Fraunhofer Diffraction Patterns
88. Physical Optics, Spatial Frequency
89. Physical Optics, Image Noise Suppression
90. Physical Optics, Image Enhancement
91. Physical Optics, Multiple Channel Information Storage
92. Physical Optics, Fourier Transforms
93. Polarization, Angle of Beam Splitter
94. Practical Applications, Burglar Alarm
95. Practical Application, Tennis Racket Performance
96. Practical Applications, Bar Code Reading
97. Practical Applications, Targeting
98. Practical Applications, Counting
99. Practical Applications, Laser Pointer
100. Practical Applications, Planetarium Pointer
101. Practical Applications, Microprojector
102. Practical Applications, Measuring the Curvature of the Earth
103. Practical Applications, Artificial Rabbit
104. Demonstration, Laser Beats
105. Demonstration, Conservation of Angular Momentum
1. Scattering Beam Visibility in Air - Shake chalk
dust in the laser beam path. The beam is in visible in a dust free room but
the particles scatter the light enabling us to "see" it. This demonstration
is most effective in a completely darkened room. Although chalk dust is very
effective, it can harm computers and other electronic apparatus in a science
classroom. You can substitute most aerosol sprays and fog from cold-mist humidifiers.
But avoid hair spray and similar materials that can clog the lungs.
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2. Scattering, Crystal Imperfection - In a darkened
room, direct a laser beam through an ice cube. The light is scattered when
the beam encounters imperfections in the crystalline structure of the ice.
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3. Scattering, Rayleigh Effect - Mix a few drops
of silver or gold nitrate in a florence flask filled with water. Polarize
the laser beam by placing Polaroid film over the front of the laser and aim
the beam down into the neck of the flask. Using a laser power meter, observe
the changes in the intensity of the scattered, polarized light when the matter
is moved in a 360 degree circle around the flask.
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4. Scattering, Size of Particles - Use the same set
up as the Rayleigh Effect above. Substitute a few drops of milk for the nitrate.
Since the particles of milk are larger than the wavelength of the laser light,
they produce a more complicated scattering pattern than that produced by the
Rayleigh Effect.
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5. Scattering, Monitoring Fluid Purity - Shine the
laser beam through a flat-sided glass container filled with a transparent
gas or liquid. Place a laser power meter on the other side of the container.
Place a small opaque object and a larger converging lens in front of the meter
detector. This will block the direct beam and focus any scattered light into
the detector. As increased amounts of colloidal particles are placed in the
container, additional light will be scattered from the main beam. High readings
on the power meter indicate high pollution.
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6. Color, Monochromaticity of Laser Light - Form
a narrow beam of white light by letting sunlight pass between two razor blades.
Allow this beam to pass through the prism. A spectrum of colors will appear.
Do the same with neon laser light. Since there are no prominent wavelengths
other than red, the laser light cannot be divided into colors. (To be precise
it must be stated that most lasers are not truly monochromatic. Metrologic
lasers, for instance, produce several closely related wavelengths).
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7. Color, Absorption of White Light - Color filters
absorb certain wavelengths, while allowing light of other wavelengths to pass
relatively uneffected. Hold filters of various colors in front of your eyes
and view objects in the room (not laser beams). Objects appear dark or black
("absence of light") when their color is absorbed.
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8. Color, Absorption of Laser Light - Filters can
act as selective absorbers of light. A filter will transmit some color and
absorb others. Most green filters are strong absorbers of red laser light.
Some colors are transmitted and other absorbed. Try some experiments using
colored cellophane or colored plastic.
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9. Color, Quantum Energy of Red Light - Shine ultraviolet
light on a zinc plate attached to an electroscope that has been given a residual
negative charge. Compare the response with that of a demonstration in which
the red laser beam is used as the light source. The electroscope discharges
immediately with the application of the dim ultraviolet light, but the intense
red light of the laser beam does not have sufficient energy to overcome the
work function of the zinc and the electroscope will not discharge.
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10. Color, Coefficient of Absorption - The thicker
the filter, the greater the absorption. Find a homogeneously colored glass
that has several thicknesses. Shine a laser beam through each of the thickness
and record the poser output using a laser power meter. Plot the power output
versus the thicknesses. The poser output should vary in an exponential manner.
For details, refer to the Beer-Lambert Law. Using inexpensive color filters,
it is relatively easy to separate the red laser light from the other wavelengths
of "white" light. Consider the bar code scanner used in supermarkets. Only
a tiny portion of the original laser beam is reflected from the symbol on
the package back to the photodetector. This small amount of light is separated
from ambient light with a red color filter. The red light passes through the
filter. The red light passes through the filter and is bright to the detector,
but the background is dark.
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11. Color, Reflection from Opaque Surfaces - Opaque
materials selectively reflect different colors. Show this by expanding the
width of the laser beam to about one-inch with a diverging lens. Hold paper
of various colors within the beam. Observe how some reflect the light better
than others do. Consider problems of a designer who must plan bar code symbols
for grocery packages. If the bars are to be printed with black ink, the background
color of the package must appear bright when illuminated with red laser light.
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12. Reflection, Specular Versus Diffuse - Objects
such as mirrors provide specular reflections, that is, they change the direction
of the beam without scattering or diffusing the light. Rough objects provide
diffuse reflections that scatter the light. Shine the laser at various objects,
and plot the reflection strength vs. angle. See how an aspirin reflects nearly
100% of the light over a large angle.
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13. Reflection, Internal Reflection - Shine the
beam into a tank of water and observe that the light will be reflected not
only when the beam enters the water, but also when the beam is leaving from
a different side of the tank. Reflections that tend to keep the laser beam
inside of the medium are known as internal reflections. The intensity of these
reflections vary with the angle of incidence as the beam attempts to leave
the medium. See if you can observe a secondary spot that a laser will generate
off to the side of the main beam. This unwanted spot is caused by reflections
inside the glass of the laser's front mirror.
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14. Reflection, Law of Reflection - The Law of
Reflection states that the angle of incidence is equal to the angle of reflection
and that the incident ray, normal, and reflected ray all lie in the same plane.
Aim the laser toward a distant mirror. Adjust the mirror so that the beam
returns to the laser aperture. Move the laser two meters to the right of the
original position. Slightly elevate the laser while aiming it at the same
spot on the mirror. Determine the relationship between the distance that the
laser was elevated and the distance that the reflected beam is depressed.
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15. Reflection, Corner Prisms - A corner prism can
be made by cutting off a corner of a box and gluing mirrors to the three inner
surfaces of the corner. Hold a flat mirror about 5 meters from the laser and
try to reflect the beam back into the laser aperture. See how much easier
this can be done with a corner reflector. The American astronauts put corner
reflectors on the moon. Now timed laser pulses from the earth can be returned
to their origin whenever it is necessary to make precise moon-earth distance
measurements.
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16. Reflection, The Colors of the Rainbow - Direct
a low-power laser into a glass cylinder filled with water. Add two or three
drops of milk or powdered cream to the water to make the beam visible. By
moving the cylinder slowly across the laser beam, the intensity of the emerging
beam will vary as the angle of incidence between the laser beam and the cylinder
is changed. The angles between the incident and emerging beam can be measured
using long distance techniques and a laser. Measurements can be related to
Sir Isaac Newton's explanation of the "Cause of Colors in the Rainbow." (See
Metrologic's "Experiments Using a Helium-Neon Laser" for Newton's explanation.)
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17. Reflection, Critical Angle - Position a laser
so that the beam enters the side of a fish tank and emerges from the top.
Fill the tank with water and a few drops of milk to make the beam visible.
Then decrease the angle that the laser beam in the water makes with the surface
until it no longer emerges but is totally reflected back into the tank. When
this first occurs, the angle between the laser beam and the normal to the
surface is called the critical angle of water. To find the index of refraction
of water, take the reciprocal of the sine of the critical angel.
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18. Internal Reflections in a Water Jet - When
a laser beam is traveling inside a stream of water, internal reflections at
the edges of the jet can prevent the beam from leaving the water. This happens
whenever the angle of incidence between the laser beam and the normal to the
water surface is greater than the critical angle for water (48° ). Punch or
drill a 5mm hole near the bottom of an empty 1 liter clear plastic soda bottle.
Aim the beam of a laser so it goes into the bottle and out the hole. Fill
the bottle with water. When the water emerges from the hole, the laser beam
will follow the water jet as it arcs downward. This illustrates the internal
reflections that take place in a fiber optics cable.
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19. Critical Angle, Minimum Radius of Curvature of Light
Pipes -
As the radius of the bed of a fiber optic light pipe is made smaller,
light begins to escape from the sides of the fiber. Determine the minimum
radius of curvature for a length of fiber optics or other light guide.
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20. Index of Refraction, Liquids - Hold a circular
protractor in a vertical position and submerge half of it in a large beaker
of liquid. Aim the laser do the beam just grazes the front surface of the
protractor and passes through its center. Measure the angles of incidence
and refraction. Calculate the index of refraction of the liquid using the
relationship n=sin i/sin r. Repeat for different angles of incidence and for
different liquids. Water, alcohol, and glycerin are suitable for this exercise.
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21. Index of Refraction, Liquid with Varying Optical
Density - If the optical density of a liquid or a gas varies, a light
beam will bend gradually as it is transmitted through the fluid. This can
be observed by partially filling a fish tank with clear water and adding several
lumps or cubes of sugar solution that is dense at the bottom and gradually
becomes less dense toward the surface.; Aim the laser beam horizontally into
the side of the tank and observe how the beam gradually bends as the index
of diffraction of the sugar solution increases. (See American Journal of Physics,
July 2 1972, p.913)
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22. Index of Refraction, Glass - When light travels
from air to glass, there is a change of speed and the beam will bend, or refract,
at the interface when it enters the glass. Measure the angle (i) between the
incident laser beam and the normal to the glass surface. Also measure the
angle ( r) between the bent beam inside the glass and the same normal. If
we assume that the index of refraction of air equals 1, the index of refraction
of the glass can then be calculated using Snells Law: N air sini = n glass
sinr
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23. Index of Refraction, Prism - When a laser beam
is transmitted through a triangular prism, the beam will be reflected twice
and emerge along a path that deviates from its original direction of propagation.
By rotating the prism, the angle deviation can be obtained is called the minimum
angle of deviation for the particular prism. By measuring the apex angle of
the deviation, the index of refraction of the prism may be calculated: Greater
precision can be obtained by allowing the beam to cross a room so that small
changes in angle will be greatly exaggerated because of distance.
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24. Variable Index of Refraction, Schlieren Effect -
Place the flame of a Bunsen, or propane, burner in an expanded laser
beam and observe the image of the flame in a nearby screen. The heated air
and rising convection currents cause variations in the index of refraction
of the air. This produces moving shadows on the screen, known as Schlieren.
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25. Index of Refraction, Gas - Arrange a Michelson
interferometer with a front surface mirror at one of the arms and a gas chamber
mirror combination at the other arm. The light from the laser is split into
two beams forming a right angle. After reflection by two mirrors, the beams
are superimposed, interfere with each other, and form a series of light and
dark fringes on a screen. Use a vacuum pump to evacuate the air in the chamber
and then allow air or other gas to return slowly. Air reduces the speed of
light and effectively increases the beam path in the gas chamber. Each time
the path increases by one wavelength, a fringe can be seen to shift on the
screen. The index of refraction of air (or any other gas in the chamber) can
be determined by using the relationship: n=( 2L + nl ) / 2 L
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26. Laser Characteristics, Beam Diameter - A laser
beam seems larger in a dark room than in a bright one. As a result, the spot
size cannot be measured by the eye or the ruler. By convention, the beam diameter
is determined by measuring between the 1/e intensity points. A knife edge
(or razor blade) mounted on a calibrated translation stage may be used to
measure beam diameter. Read the full beam power with a laser power meter.
Move the knife edge into the beam until the power drops to 90 percent of full
power. Note the micrometer reading of the stage. Then move the knife edge
further into the beam until the power drops to 10 percent of full power. Note
the micrometer reading. The difference between the two readings divided by
.65 is equal to the beam diameter.
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27. Laser Characteristics, Beam Divergence - Divergence,
the angle at which the beam expands, is usually measured in milliradians and
expressed as the full angle, 2q . Divergence should be measured several meters
from the laser because the function is non-linear close to the laser aperture.
To find the divergence in radians, divide the beam diameter by the distance.
(e.g., 1.5 cm diameter divided by a 1,000 cm distance equals 1.5 milliradians.)
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28. Laser Characteristics, Polarization - Helium-neon
lasers that do not have Brewster windows at their ends emit light that is
randomly polarized. At a given instant, such polarization may be described
by two ellipses, each at right angles to the other. Over a period of time,
the resultant ellipse will change in orientation and relative magnitude, while
maintaining a constant power. Neither power meters nor the human eye can detect
these changes in polarization. But, if a polarizing filter is placed in front
of the laser power meter, the filter will favor one polarization component
and block the other. The power readings will vary in response to the changes
in relative power between tow polarization components. This effect is best
observed while a short laser is warming up.
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29. Optics, Multiple Beams - A single laser beam
can be split into a number of laser beams by directing it through several
microscope slides. Tape the sides together on one end. One the other end,
wedge a small piece of cardboard between each slide and then tape the slides
together.
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30. Optics, Cylindrical Lens - A glass stirring
rod serves as an inexpensive cylindrical lens. It will spread the beam out
so that it forms a lie of light rather than a round spot. This technique is
used in sawmills to provide a guideline for cutting lumber.
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31. Optics, Ray Paths - Use a laser to study geometric
optics. Its intense beam indicates ray paths clearly. To make the laser beam
visible to a large group, sweep a white card along the beam path. Angle the
card toward the viewers. In a container of water, add drops of milk to view
the laser beam.
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32. Geometric Optics, Lens Focal Length - Use two
lasers or divide the beam of a single laser into two parallel components.
Pass the beams through a lens and determine the distance beyond the lens at
which the beams converge. Alternatively, expand and collimate the beam using
a beam shaping telescope. Observe the cone of light as the lens focuses it.
Use the techniques in No. 31 to make the beam visible.
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33. Optics, Beam Waist - Wave optics demonstrations
of the laser beam waist can be performed by passing the light through a telescope
or a pair of lenses so that it focuses at a distance. Wave a card along the
beam path and notice how the beam radius narrows as the beam waist approaches.
The beam will be symmetrical on each side of the beam waist.
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34. Beam shaping, Magnification - Pair several
different lenses to make beam shaping telescopes of several magnifications.
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35. Beam Shaping, Lens Focal Lengths - The focal
length of a compound lens made of two simple lenses may be predicted if we
know the focal lengths of each lens and the distance between them. If known
values for these parameters are substituted in the lens maker's formula, the
combined focal length of the two lenses may be predicted. If either of the
two lenses is divergent, use a negative focal length for this lens when substituting
the value in the formula. If the calculated answer comes out with a negative
number for the combined focal length, it indicates that the lens system has
a virtual rather than a real focus. Check the accuracy of your predictions
by putting the lens system in a collimated laser beam.
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36. Beam Collimation - Although a laser beam is
essentially parallel and does not diverge substantially, over a distance of
several hundred meters the amount of spreading becomes significant. To minimize
adverse effects of this spreading, collimate the beam. This is done by first
spreading the beam with a diverging lens and then making the edges of the
beam parallel with a converging lens. By adjusting the distance between the
two lenses, the beam can be collimated so it remains at a constant width of
about 2cm over distances of several kilometers.
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37. Beam shaping, Depth of Field and Beam Waist Diameter
- TEMoo mode lasers have beams that converge to a minimum radius, called the
beam waist, and then expand. Collimation is maintained for a short distance
in the beam waist that may be called the depth of field. Each end of the depth
of field has a diameter that is 1.4 times that of the beam waist. Beyond the
end point, the beam begins to diverge and is no longer collimated. If the
beam waist is broader, the depth of field will be longer. These values are
theoretical limits based on l/e2 diameter and 633 Nm light
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38. Diffraction, Knife Edge - The laser offers
convincing proof that light actually bends, or is diffracted, around small
objects. Demonstrate knife edge diffraction by pointing the laser at a screen,
such as a sheet of glossy white paper, approximately three meters away. Slide
the edge of a new razor part way into the laser beam and observe the interference
patterns on the screen. Close observation will show that there is no sharp
shadow on the edge of the razor blade on the screen, but instead there is
a diffraction pattern consisting of a series of light and dark fringes parallel
to the edge of the razor blade. Make a graph of the intensity variations of
these fringes by moving a photometer across the screen while recording the
intensity variations. Compare this graph with the theoretical patterns that
can be found in a textbook on optics.
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39. Diffraction, Narrow Slit - Using the same setup
as for the knife edge diffraction, insert a second razor blade into the other
side of the laser beam so that the edge diffraction patterns produced by the
two razor blades are superimposed. Observe the effects as the razor blades
are brought closer and closer together to form a narrow slit. Observe the
variations in the intensity of the fringes and the distance between them as
the razor blades are brought closer together.
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40. Diffraction, Variable Slit - Make an inexpensive
variable slit using a vernier caliper and a double edge razor blade. Bend
the blade until it snaps along the center and breaks into two halves. Glue
or tape each half to a jaw of a bernier caliper as shown in the diagram below.
Aim a laser beam at the gap between the blades and observe the single slit
diffraction pattern on a distant screen. If the width of the slit is known,
the wavelength of laser beam can be determined. Alternately, by using a laser
beam of known wavelength, the width of the gap between the jaws can be measured
with a high precision.
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41. Diffraction, Double Slit - When the laser beam
is sent through two narrow parallel slits, each slit produces an identical
diffraction pattern. If the slits are close together, the diffraction patterns
overlap and a phenomenon called double slit interference occurs. Investigate
the characteristic diffraction and interference patterns as the width and
spacing of two slits are varied. Precision double slits for this investigation
can be found on the Cornell diffraction slit plate or the Metrologic diffraction
slide mosaic furnished with the laser Optics Lab Kit. Make a sketch of a typical
diffraction pattern and explore the intensity of illuminations at various
locations using a photometer.
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42. Diffraction, Multiple Slit - When laser light
is transmitted through a series of narrow parallel slits that are spaced close
together, each slit produces a diffraction pattern that overlaps the patterns
produced by the others. If the slits are evenly spaced, a pattern that is
very similar to that of the double slit diffraction results, but the additional
light coming from the multiple slits produces a much brighter pattern (see
Metrologic's Experiments Using A Helium-Neon Laser.)
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43. Diffraction, Small Holes - Diffraction of a
laser beam by a small hole is due to the interference of diffraction fringes
from the edges of the hole causing interference in the overlap area. Make
pinholes in a square of aluminum foil using a sharp needle. For better pinholes,
pull a sheet of fresh carbon paper through a gap between a tesla spark coil
and a ground plate. When the laser beam goes through the hole, a bullseye
airy disc pattern will appear on a distant screen. If the hole is good, the
pattern will be perfectly round. Experiment making the pinholes smaller. The
smaller the hole, the larger the bullseye. Calculate the size of your pinhole
by measuring the radius of the Airy disc in the bullseye pattern. Appropriate
formulas for this calculation are given in the Metrologic book, Experiments
Using a Helium-Neon Laser, and in many optics text books. The same technique
can be used to measure the diameter of a blood cell. (see No. 80).
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44. Diffraction, Reflections from a Ruler - Diffraction
may be observed by reflections from a steel machinist's ruler or an ordinary
plastic ruler with fine black markings spaced at millimeter or sixteenths
of an inch intervals. Mount the ruler in the laser beam path as shown. The
beam should just graze the ruler so that it illuminates approximately three
centimeters of the scale. Place a screen about two meters away from the laser
so that the diffraction pattern from the ruler may be observed. The pattern
will consist of alternate light and dark areas spaced at regular intervals.
Small adjustments of the ruler with respect to the laser beam may be necessary
to sharpen the pattern. Place a diverging lens in the laser beam at the end
of the ruler so that fine details of the diffraction pattern may be observed
on the screen. When this has been done, move the ruler slightly while watching
the screen. Try to explain why the diffraction patterns change the way they
do.
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45. Interference, Thin Film - When two sheets of
flat glass are separated by a thin film of air, multiple reflections of light
occur at the air-glass interfaces. If the thickness of the film is adjusted
so that it is 1/4 the wavelength of the incoming light (or odd multiples of
1/4 wavelength), most of the light will not be transmitted, although air and
glass are transparent. Shine a laser beam through the air wedge supplied in
the Metrologic Laser Optics Lab kit. Enlarge the beam using a diverging lens
and view the resulting interference patterns on a screen. Vary the thickness
of the air film edge by squeezing the glass plates together at various places
around the edge. Observe changes in the pattern on the screen as the glass
is squeezed.
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46. Interference, Multiple Internal Reflections in
Glass - When a laser beam is transmitted through a flat glass plate,
some light is transmitted and some light is transmitted and some is internally
reflected in the glass. As the reflected light subsequently emerges from both
sides of the microscope slide at intervals along the surfaces, interference
occurs among the multiple beams of coherent light. Interference patterns may
be obtained by placing the microscope slide in the beam, as shown in the diagram,
so that it is almost parallel to the beam. Try to explain why one of the patterns
is the reverse of the other.
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47. Interference, Window Glass - Shine a laser on
a piece of glass and allow the reflected light to cross a darkened room. An
interference pattern will be observed on the far wall. The fewer and more
parallel the interference bars, the more uniform the glass thickness.
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48. Interference, Heat Expansion - Touch a soldering
iron to the piece of window glass used in the last exercise. A localized distortion
of the interference patterns will appear as the glass expands after being
heated with the tip of the soldering iron.
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49. Interference, Evaporation of Alcohol - Place
a lens within a laser beam and project the enlarged image onto a viewing screen.
Place a drop of alcohol on the lens and allow the alcohol to evaporate. As
the evaporation proceeds, changing interference patterns will be observed
on the screen.
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50. Interference, Michelson Interferometer - Dimensional
differences less than 1/4 wavelength wide can be observed using an interferometer
made with a beam splitter, two mirrors, and lens. The beam splitter divides
the beam and sends it in two separate directions. Two mirrors return and combine
the light. When the light waves combine, they overlap and interfere with each
other. When enlarged through a lens and projected on a screen, the canceling
effect can be seen as alternate light and dark bands or interference fringes.
Observe how the fringes shift when a person coughs of shouts into the apparatus.
The beam splitter, front surface mirrors. Lens and magnetic holders for the
optics components are included in the Metrologic Laser Optics Kit.
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51. Interference, Changes in Volume - Minute changes
in volume during heating or cooling can be observed in the classroom. For
example, if one of the mirrors in the above experiment is heated with a hair
dryer, the fringes will shift. Commercial uses of interferometry include:
distance, contour, thickness, and flatness measurements; inspection of optical
material; determination of the index of refraction of glasses and gases; and
examination of dynamic changes in wind tunnels. These applications are especially
useful in student research projects.
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52. Interference, Lloyd's Mirror - Lloyd's mirror
is a classic optics experiment and was first described in 1834. Place a converging
lens about 10cm in front of a diverging lens. Shine laser into both. Place
a screen about three meters from the laser and adjust the position of the
two lenses so that the smallest possible spot can be seen on the screen. Lay
a microscope slide between the two lenses. Carefully raise the slide until
the laser beam just grazes its upper surface. A second spot will appear on
the screen, about one inch above the first. Remove the converging lens without
disturbing the other apparatus. With this lens removed, the cones of light
coming from the direct and reflected sources partially overlap, forming an
interference pattern on the screen. Calculate the wavelength of the laser
light using the formula for double slit diffraction. Further details to perform
this experiment can be found in the Metrologic book, Experiments Using Helium-Neon
Laser.
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53. Modulation, Morse Code Communication - Aim
a laser beam at a distant solar-cell photodetector, which is connected to
the microphone input of an audio amplifier. Whenever the laser beam is interrupted,
the amplifier-speaker will produce a loud click. Make a Morse code "dot" by
passing your index finger through the laser beam. Make a "dash" by spreading
out four fingers on your hand and passing them through the beam at a uniform
rate.
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54. Modulation, Voice Communication - Plug a microphone
into a modulated laser. Talking into the microphone alters the laser tube
current and the beam brightness varies according to speech patterns. When
the light from the beam reaches a distant photodetector, the electronic waveforms
that are produced can be amplified to reproduce the original sounds. The Metrologic
Speed of Light/Laser Video Kit contains an appropriate photodetector, amplifier
and speaker for this particular application.
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55. Modulation, Video Transmission - Connect the
output of a TV camera or video recorder to the video jack at the back of a
modulated laser. The video signals will vary the intensity of the laser beam.
Set up a distant video receiver(wide-bandwidth phototransistor and video amplifier)
to receive the modulated laser beam and operate a black and white or a color
TV monitor to produce the images. Full resolution cannot be attained using
a helium-neon laser because its bandwidth (about 0.6MHz) is smaller than that
of a TV signal (4.2MHz). However, a solid-state VLD laser is more than adequate
for the purpose.
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56. Speed of Light, Modulation Method - Use a modulated
laser to measure the speed of light across a football field and back. Set
up a distant mirror at the far end of the field and partially insert a small
mirror into the beam path near the laser to isolate a small percentage of
the light. Direct the reflected light from each mirror into a separate photodetector
placed near the laser. If the photodetectors are connected to an oscilloscope
and a rf signal generator is used to modulate the laser, the resultant of
two waveforms will be displayed on the oscilloscope screen. Because of the
time delay to send the light over the long leg, the two photodetector inputs
will be slightly our of phase. The phase difference between the two waveforms
can be measured and equated to the difference in travel time of the two beams.
The speed of light can be calculated using this time and the distance.
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57. Holography, Observing Holograms - In a darkened
room, expand a laser beam with a diverging lens. Hold a hologram is the laser
beam path and allow the laser light to enter your eyes. Search for the holographic
image within an angle of 20-40 degrees to the right or left of the laser but
DO NOT stare directly into the laser.
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58. Holography, Making White Light Reflection Holograms
- Cut a small piece of non-AH (anti-halo) holographic film into a two
inch square. Place the film between two pieces of glass. Hold the glass together
using two ordinary binder clips. Use a diverging lens to expand a laser beam.
Direct the beam through the film onto an object placed directly behind the
film. The hologram is formed between the reference beam coming from the laser,
and the object in white light (See Metrologic's book Holography Using a Helium-Neon
Laser written by Dr. Tung Jeong for details).
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59. Holography, Cylindrical Holograms - A 360-degree
view of an image can be obtained by strapping a hologram inside a glass cylinder,
emulsion side inward. Wrap a piece of film, emulsion side inward, inside a
cylinder. Use a diverging lens to expand a laser beam. Shine the beam into
the top of the cylinder and observe the image through the glass sides.
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60. Holography, Transmission Holograms - The best
depth of field and sense of perspective is obtained with transmission holograms.
A recommended setup to make one requires the splitting of a laser beam. Allow
one beam to go directly to the film. Direct the second beam at the object,
and then to the film. This type of hologram can only be seen using laser light.
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61. Hologram, Multiple Channel - More than one
picture can be stored on holographic film. A simple method is to make the
first exposure, then change the angle of the film plate and make a second
exposure using a second object. Two separate pictures can be seen by tilting
the film plate. (See Holography Using a Helium-Neon Laser for further details).
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62. Holography, Holographic Interferometry - Holography
is widely used in industry to observe minute changes in materials. The techniques
involves making two exposures of the same object; between exposures, the object
is moved or changed. The resulting hologram shows the object covered by light
and dark interference fringes, which indicate a 1/4 wavelength change. For
example, shoot a C-clamp, tighten it slightly, and shoot again. Interference
fringes will show where the clamp is under the greatest stress (See Holography
Using a Helium-Neon Laser by Dr. Tung Jeong).
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63. Applications, Pipe Laying - Most major sewer
and storm pipe installations are aligned using helium-neon lasers. A laser
beam is aimed down the center of a pipe. As each successive pipe is connected,
a round disc with a center hole is placed over the end of the pipe. The pipe
is moved slightly until the laser beam shines through the central hole. This
provides fast and accurate alignment and cost-effective drainage.
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64. Applications, Optical Lever - Attach a mirror
to an object. Aim a laser beam at the mirror and allow the reflected beam
to fall on a distant screen. Any small movements or changes in the object
will cause a large displacement of the red dot on the viewing screen.
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65. Applications, Optical Galvanometer - Wind 20
turns of insulated wire around a screwdriver handle. Remove the screwdriver
and tape the coils together. Attach two thin strands of wire to the ends of
the coil. Using a small piece of cellulose or double-face tape, fasten the
coil to a thin mirror. Suspend the coil and mirror between the poles of a
horseshoe magnet. Aim a laser at the mirror and observe its reflection on
a distant wall. Do not stare at the laser beam or its bright reflection. Apply
a small current to the coil and the mirror will twist, resulting in a laser
beam deflection. Using the appropriate shunts and multipliers, calibrate the
galvanometer to read amperes and volts. An ammeter or voltmeter may be used
for comparison.
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66. Applications, Pulse Indicator - Use adhesive
tape to fasten a small mirror to your wrist directly over the point where
the pulse id found. Aim a laser at the mirror and observe its reflection on
a wall. Even when holding your wrist steady on a table, small movements in
the mirror due to your pulse will be observed as large deflections on the
wall.
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67. Applications, Laser Art - Many laser lighting
effects are created by sending laser light through transparent materials,
such as reticulated plastics that are used as diffusers in fluorescent ceiling
lights. The fine mesh cloth used in silk screen printing produces interesting
symmetrical arrangements of sots of light. Low cost diffraction grating can
be used to split the beam into dots arranged in a single straight line. For
slowly changing patterns, aim the laser beam through transparent liquids,
such as oil and water in a shallow glass container.
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68. Applications, Inspecting Wobble or Height -
Expand a laser beam to a diameter larger than the anticipated change in wobble
of height. Direct the laser beam across the edge of an object, and then into
a lens that will focus the beam to the photocell of a laser meter. The power
at the detector will vary depending on how much of the beam is obstructed
by movement of the object.
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69. Applications, Measurement, Wire Diameter -
Laser light diffraction is being used in industry to measure the thickness
of fine wire. Direct a laser beam at a filament, such as a human hair. Measure
the spacing between the light and dark areas of the diffraction pattern and
calculate the diameter of the hair using the standard diffraction formula.
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70. Applications, Foucault Knife Edge Test - The
Foucault knife edge test is a standard technique used in the optics industry
to test the quality of lenses and curved mirrors. Use two lenses to collimate
a laser beam. Place the lens to be tested in the beam path. Insert a razor
blade at the focal point of the lens being tested an view the results on a
screen. If the lens is perfect, the spot on the screen will darken uniformly
as the razor blade cuts the beam.
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71. Applications, Spatial Filter - A spatial filter
cleans a laser beam by removing optical noise. Direct a laser beam at a converging
lens, and then center a pinhole at the apex of the cone of light produced
by the lens. The pinhole should be large enough to let the primary beam pass
through easily, but small enough to block any stray light. (Suitable pinholes
can be made by folding several thicknesses of aluminum foil, laying them on
a piece of glass and puncturing them with a pin. The best pinholes are found
in the middle folds.) It is critical that the pinhole be placed at the precise
focal point. This requires adjusting the pinholes in three directions, along
the X, Y, and Z axes. Most of the optical noise will be blocked by the material
surrounding the pinholes.
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72. Applications, Rangefinding - Use two lasers
(or use only one laser with a beam splitter and a mirror) to generate two
laser beams that are separated by at least 50cm. Leave one of the lasers fixed,
and rotate the other laser so that the beams intersect on distant objects.
Use triangulation trigonometry or a tape measure to calibrate the rotation
angles that are required to target objects in terms of their distance from
the lasers.
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73. Applications, Wheel Alignment - Mount a mirror
in the center of a wheel so its surface is perpendicular to a laser beam.
Rotate the wheel and observe the reflected light on a distant wall. If the
wheel or shaft is out of alignment, the reflected light forms a circle. If
the wheel or shaft is properly aligned, the reflected light comes to a point.
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74. Applications, Laser Oscilloscope - Using the
principles of optical amplification, Lissajous patterns can be projected on
a distant wall and the principles of an oscilloscope can be demonstrated.
Mount two 3-inch radio speakers in a frame as shown in the diagram. Glue one
end of a short wooden dowel to the center of each speaker. Glue the other
end of the dowel to a thin strip cut from a plastic ruler; attach the free
ends of the plastic strips to the frame. Cement a small mirror over the area
where the plastic strips intersection. Aim the laser at the mirror and observe
its reflection on a distant wall. DO NOT stare directly at the beam or its
bright reflection. Connect an audio oscillator to speaker 1 and another speaker
2. Adjust the oscillator frequencies to produce a variety of large Lissajous
patterns on the wall. Horizontal deflections of the beam are produced by speaker
1. Vertical deflections of the beam are produced by speaker 2.
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75. Applications, Detect Flaws - Shine a laser
beam on a surface to detect flaws and scratches in a material. Scattering
of the light will occur where a flaw or scratch is present.
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76. Applications, Lissajous Patterns - Among the
most well-known laser lighting effects are Lissajous figures, a swing pattern
of interwoven circles which have become almost a trademark of laser light
shows. These intricate patterns can be created easily. Mount small mirrors
on the rotor shafts of two variable speed motors. Position each mirror at
a slight angle so that when the shaft rotates, a small circle is reflected.
Place the laser and the two motor/mirror assemblies so that the laser beam
follows a Z-shaped path, from the laser to one mirror, to the second mirror,
and then to a wall or screen. If the speed of each motor can be adjusted independently,
a great variety of Lissajous patterns can be created.
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77. Applications, Wave Guides - A look into the
future shows small, solid state lasers producing an optical signal that will
be carried miles by a fiber optics wave guide. Low loss wave guides will be
a necessity to produce an effective laser communication system. To make a
low loss wave guide, fill some quartz tubing with purified trichloethylene.
Losses less than 20 dB/km have been reported using this liquid core wave guide,
although losses of more than 1.2000dB/km are usually found with conventional
glass fibers. Try making your own wave guides and measure the losses with
the aid of a laser power meter.
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78. Applications, Measurement by Scanning - Mount
a mirror or a many-sided prism on a motor that rotates at a constant speed.
Direct the laser at the mirror and place a photodetector in the reflected
beam scans the target and its width can be measured by the time that the widths
of the bars on bar code symbols to the thousandth of an inch.
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79. Applications, Ophthalmology - When an enlarged
laser beam is aimed at a wall, the illuminated area appears to have small
spots or grains. This granular appearance is caused by a complex interference
pattern produced by the coherent light as the lens of our eye focuses it on
the retina. Expand a laser beam with a lens and project it on a wall or screen.
Move your head slowly from side to side while observing the spot. If your
are farsighted or your eyesight is normal, the spots will move in the same
direction as your head. If you are nearsighted, the spots will appear to move
in a direction opposite from that of your head. In nearsighted people, the
eye tends to focus the pattern a short distance in front of the retina. Therefore,
parallax caused by the head movement results in an apparent motion of the
spots in the opposite direction.
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80. Application, Measure Blood Cell Diameter -
Use the Airy disc principle in No. 42 to measure the diameter of a blood cell.
Place a drop of blood on a clean slide. Gently spread the sample across the
slide using the edge of a second slide, being careful not to crush the red
blood cells or destroy their circular symmetry. Place the slide within a laser
beam, then move the slide around until a good set of diffraction rings is
observed on a screen. Measure the distance from the cells to screen and the
diameter of the first fringe. Calculate the diameter of the blood cell using
the technique reported by Dr. James E. Parks of Western Kentucky University.
Details can be found in Metrologic's book Experiments with a Helium-Neon Laser.
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81. Applications, Doppler Effect - The Doppler
effect is a well-known phenomenon by which wave frequency is changed by a
moving object. Use a Michelson interferometer setup. Superimpose the two reflected
beams on a Speed of Light Kit. Move one of the front surface mirrors back
and forth slowly. This will produce an audible tone from the loud speaker
because the wavelength of light is changed. Rapid movements, however, produce
frequency changes that are supersonic and cannot be heard.
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82. Applications, Calibrating a Diffraction Grating
- Shrinking or expanding of the film or other diffraction grating media can
change the line spacing of a diffraction grating. Always check the calibration
of a new diffraction grating with the aid of a helium-neon laser as follows:
Align the beam of a helium-neon laser so it turns right angles with the surface
of a distant screen. Tape the diffraction grating over the laser aperture.
Observe the central maximum spot and spots from other orders that appear on
the screen. Calculated (the distance between diffraction grating lines) using
the equation l =d sin q .
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83. Applications, Measuring Wavelength of a VLD Laser
- The wavelength of a VLD laser can be anywhere from 660 to 680nm depending
on the geometry and temperature of the individual laser chip. To find its
precise wavelength: Calibrate a diffraction grating using the procedure given
in the item above. Align the beam of the VLD laser so it forms right angles
with the surface of a distant screen. Tape the calibrated diffraction grating
over the laser aperture. Observe the central maximum spot and the first order
spots that appear on the screen. Find the wavelength using the equation l
= d sinq
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84. Applications, Leveling Optical Benches - Because
the bright red beam of helium-neon laser can indicate the light path, it can
be used to align lenses and other optical components. See Metrologic's Experiments
Using a Helium-Neon Laser for instructions on leveling optical benches and
centering lenses based on procedures contributed by Martin Dvorin of Monroe
Community College.
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85. Measuring the Speed of Light, Mechanical - One
method to measure the speed of light along a short path of travel (about 30
meters) is to use a rotating mirror. If the mirror is rotating at about 500
revolutions per second, a beam traveling about 40 meters will be displaced
about 4mm when measured on a screen. Use lenses in the optical system to focus
the laser beam on a viewing screen for precise measurements. Details can be
found in Metrologic's Experiments Using a Helium-Neon Laser. Dick Pontine,
of Hamline University, reports measurements having less than 3% error using
various distances from 20-60 meters.
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86. Physical Optics, Fraunhofer Diffraction - Form
a collimated laser beam about 1 inch in diameter. Place transparencies of
test patterns within this beam. Place a converging lens one focal length beyond
the transparency and a viewing screen one focal length beyond the lens. The
Fraunhofer diffraction pattern observed on the screen will look like the object.
For details see Physical Optics Experiments Using a Helium-Neon Laser written
by Arthur Eisencraft and published by Metrologic.
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87. Physical Optics, Filtering Fraunhofer Diffraction
Patterns - Follow instructions in No. 84, except remove the screen. The
focal point where the screen was located is the diffraction plane. Beyond
the diffraction plane, add another lens to collimate the light and to project
it to a screen. Place various masks or filters at the diffraction plane and
observe the portions of the patterns that are transmitted to the screen.
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88. Physical Optics, Spatial Frequency - Place
a variable diameter iris at the diffraction plane described in No.85. An increase
in the size of the iris permits more of the diffraction pattern to contribute
to the image. Experiment with various object transparencies and discover that
widely spaced lines are encoded near the center of the iris, while tightly
spaced lines are encoded far from the center. As the iris is opened, higher
spatial frequencies are transmitted, adding detail to the image.
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89. Physical Optics, Image Noise Suppression -
Follow the instructions in No.84. A mask similar to the one illustrated here
will help suppress horizontal information.
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90. Physical Optics, Image Enhancement - Follow
the instructions in No. 84. A continuous tone, photographic image can be recovered
from a halftone dotted image by placing a small aperture at the diffraction
plane.
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91. Physical Optics, Multiple Channel Information Storage
- Photograph a picture or image modulated by horizontal lines. Expose
it again with an image modulated by vertical lines. The double exposure now
stores two images. Either of the images can be retrieved by viewing the composite
using the appropriate mask at the diffraction plane.
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92. Physical Optics, Fourier Transforms - The
diffraction patterns observed using Metrologic's Physical Optics lab are examples
of Fourier analysis of the slit or other functions. The masks in the lab compute
and display the Fourier transform at the speed of light.
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93. Polarization, Angle of Beam Splitter - Place
a glass microscope slide in the laser beam at an angle so that the beam will
be reflected 10 degrees from normal. Observe variations in the intensity of
the transmitted and reflected beam over a 1 to 2 minute period. Repeat by
rotating the slide in 10 degrees steps until the slide has been rotated 90
degrees. At what angles are the fluctuations in intensity prominent? Are these
angles related to Brewster's angle? The fluctuations occur because the beam
splitter serves as a polarizer at certain angles. Beam splitters reflecting
at small angles, such as 10 and 20 degrees, will not serve as polarizers.
This experiment is best performed while the laser is warming up. After the
warm-up period the laser's polarization takes longer to change.
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94. Practical Applications, Burglar Alarm - A laser
may be aimed through a window to the outdoors and then reflected as desired
by a series of mirrors to form an invisible fence that ends in a photodetector.
Whenever the beam is broken the detector signal operates any conventional
burglar alarm siren or communication device.
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95. Practical Application, Tennis Racket Performance
- Secure a tennis racket to a table using a heavy vise or C-clamp. Place
a ball on the racket and aim a laser beam so it just grazes the top of the
ball as shown in the diagram. A photodetector connected to an oscilloscope
produces a trace which can show the initial velocity, ballstring contact time,
and the rebound velocity. For further information, see "Physics of the Tennis
Racket" in the June 1979 issue of the American Journal of Physics. Contributed
by Howard Brody, University of Pennsylvania, Philadelphia, Pennsylvania.
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96. Practical Applications, Bar Code Reading -
Bar code data on grocery products can be decoded because the bar code reflects
laser light in a distinctive pattern from the contrasting bars and spaces.
The reflected light is received by a photodetector and the signals are converted
into digital data that can be processed by a computer. The Universal Products
Code (UPC) shown on the diagram below starts with four long bars to initiate
the reading sequence, the number 2800 which is the manufacturer's identification,
0933 which is the number of the product, and four long bars to end the sequence.
The long bars at the beginning and end differ slightly in their width and
spacing. This tells the computer to make an automatic correction if the bar
code is read from the reverse direction.
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97. Practical Applications, Targeting - A narrow
collimated laser beam can pinpoint targets and help align apparatus in a very
short time. One classic example is the "Monkey and Hunter" demonstration.
A laser is aimed through a blowpipe gun at a monkey target suspended from
the ceiling. After the alignment, the "Hunter" operates the blowpipe, sending
a missile towards the target. At the same instant an electromagnet releases
the monkey. Since both the missile and the monkey fall with the same acceleration,
the missile does not miss the target.
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98. Practical Applications, Counting - Everything
from freeway traffic to items on a conveyor line can be counted as they break
a laser beam set up between a laser and a photodetector. An electronic calculator
can be modified to record a count each time the beam is broken. This technique
can also be used with ordinary light, but the laser is preferred because its
narrow beam does not require focusing and red filter minimizes interference
from ambient while light sources.
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99. Practical Applications, Laser Pointer - Use
the red beam as a pointer for slide shows or lectures in which you want to
call attention to areas on maps, charts, or photographs. The laser produces
a small bright spot that can be clearly seen on a brightly illuminated projection
screen. Furthermore, the beam does not have to be focused each time the distance
from the lecturer to the screen changes.
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100. Practical Applications, Planetarium Pointer -
Use the Metrologic VLD laser ML268 to point out stars, constellations
and other features that are projected on a planetarium dome. Its major advantage
over a flashlight arrow pointer is that the narrow red beam is easily seen
on the planetarium dome, but its brightness does not overpower and obscure
nearby stars.
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101. Practical Applications, Microprojector - Shine
the laser at the substage mirror and remove the eyepiece of a microscope to
use it as a microprojector. A screen placed at the focal plane of the objective
lens will produce an enlarged image of the specimen on the stage so several
people can view it simultaneously.
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102. Practical Applications, Measuring the Curvature
of the Earth - Set the laser on a tripod a short distance above the ice
on a large frozen lake. Collimate and aim the beam horizontally over the ice
with the aid of an accurate bubble level. Several kilometers away set up a
telescope to intercept the laser beam. Because of the curvature of the earth,
the height of the telescope above the ice will be greater than that of the
laser. By measuring the difference in height between the laser and the telescope,
the size of the earth can be calculated. If there is no ice, try it using
boats on a day when the water is mirror calm.
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103. Practical Applications, Artificial Rabbit -
This last application is a bit frivolous but it usually works and might possibly
be useful to someone. On a dull day of after the sun has set, aim your laser
out the window so it makes a small spot on the grass or bushes outside. If
the beam is moved about, most dogs will run as fast as they can trying to
catch the spot.
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104. Demonstration, Laser Beats - To visualize
the phenomena of beats and intensify the sound for a large audience, use the
setup shown in the diagram below. The tuning forks have polished prongs and
are mounted on resonance boxes. Adjust the frequencies of the tuning forks
with sliding weights so the difference is 10Hz or less. The reflected beam
can be displayed on a screen, generate waveforms on an oscilloscope, or can
be detected and amplified to be heard in a large lecture room. Contributed
by Ellis D. Noll, Conrad Weisner High School, Robesonia, Pennsylvania.
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105. Demonstration, Conservation of Angular Momentum
- Glue a tiny mirror chip to the side of a stopwatch. Balance the stopwatch
on top of an inverted watch glass placed on the table. Aim a laser beam at
the stopwatch mirror so its reflection produces a small spot on a screen about
2 meters away. When the watch is running, a streak of light is seen on the
screen. Its length is proportional to the angular displacement of the watch.
It is also possible to compute the frequency of oscillation (5Hz), the period,
the angular velocity, and the angular momentum using the setup shown below.
Contributed by Arthur R. Quinton, University of Massachusetts, Boston, Massachusetts.
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