Optics: "Laser Light Show"
Martin Liphardt
Edited by Anne Starace
Abstract
This module demonstrates the principle of how the amazing
figures in Laser light shows are generated. The red light from a
small diode Laser is aimed at a mirror that is glued to a powered
speaker. The reflected light is visible as a small spot on a distant
wall/screen. Sound collected by a microphone causes the
speaker to vibrate, which generates interesting patterns by the
Laser spot on the wall/screen.
Caution!
This demonstration uses a Laser.
Read the safety instructions before using this demo.
Funded
by
the
National
Science
Foundation
 1997 University of Nebraska. All rights reserved.
Laser Light Show: version2.0
and
the
University
of
Nebraska
1
Content Standards
K
1
2
3
1.2.1
4
5
6
7
4.2.1
8
8.2.1
4.3.3
History & Process Standards
K
1
2
1.6.1
3
4
4.6.1
5
6
7
8
8.6.1
Skills Used/Developed:
 1997 University of Nebraska. All rights reserved.
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TABLE OF CONTENTS
I. OBJECTIVES...............................................................................................................................4
II. SAFETY......................................................................................................................................4
III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS.......................................4
IV. LIST OF MATERIALS.............................................................................................................5
V. INTRODUCTION ......................................................................................................................6
VI. PROCEDURE............................................................................................................................7
A. Setup ........................................................................................................................................7
B. Execution ...............................................................................................................................10
C. Cleanup ..................................................................................................................................10
VII. FREQUENTLY ASKED QUESTIONS ................................................................................11
VIII. TROUBLESHOOTING........................................................................................................11
IX GLOSSARY OF TERMS.........................................................................................................11
 1997 University of Nebraska. All rights reserved.
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I. OBJECTIVES
Students will observe the sep up of a laser light show.
II. SAFETY
Important! Please read!
Safety is an important issue in this demonstration because it involves the use of a laser. The
laser(s) used in this demo are potentially harmful to the human eye even though their output
power is less than 5 mW. They are potentially harmful because the output power is spread over
a very small area compared to a number of other light sources. This causes the light intensity,
the power per area, to be much higher. Here is an example:
Laser
Light Bulb
(rated ~ 50 W)
Flash Light
Light Power (in Watts)
0.005
5
1
Area covered by light
(in m2)
0.000005
(5*10-6)
50 (at 2 m)
0.01 (at 10 cm)
Intensity (Watts/m2)
1000
0.1
100
As you can see the intensity of the light from the laser is much higher than that of the light bulb
at a distance of 2 meters. The light from a flash light at short range is also relatively high. It is
intense enough to cause us to see stars for a while when directed into the eye. The laser light is
more intense and will certainly cause more harm than that. So be careful!
IF YOU BRING ALONG ANOTHER LASER MAKE SURE YOU KNOW ITS OUTPUT
POWER RATING AND ITS LASER CLASS RATING. IF YOU DO NOT UNDERSTAND
THIS DO NOT BRING ANOTHER LASER.
III. LEVEL, TIME REQUIRED AND NUMBER OF
PARTICIPANTS
A. LEVEL
There is no minimum age recommendation. There is no recommended upper limit, although
older high school students may not be as interested if they have already seen similar
demonstrations.
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B. TIME REQUIRED
25 minutes.
C. NUMBER OF PARTICIPANTS
Not limited. Small (<10) and large (>50) groups possible.
IV. LIST OF MATERIALS
•
•
•
•
•
Diode Laser w/ 4 AA batteries
Laser holder (third hand)
Walkman w/ stereo cable
Powered Speaker w/ mirror
Microphone
 1997 University of Nebraska. All rights reserved.
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V. INTRODUCTION
Laser Light Shows have become very popular. The principal of producing these nice figures is
not too difficult to understand although it takes a bit of technical and choreographic expertise to
create a good Laser Light Show. As you may imagine, Lasers are needed for a Laser Light
Show. Light from a Laser comes in many different (pure) colors, and it usually comes in the
form of a narrow beam. Such narrow beam produces only a small color spot on a screen, like the
tip of a color pen or marker. The difference between a Laser and a pen is that when the Laser
light spot is moved across the screen it does not leave a trace behind it.
So how can we use this light spot to draw figures?
The answer has to do with how the human eye and the human brain perceive images. Each
image stays in the brain for a short time. So, if the laser spot is moved at high speeds it actually
leaves a trail (in peoples’ minds) that disappears after this short time. The secret of making this
trail appear to be permanent is to repeat the motion of the laser spot before the trail disappears
and then repeat it again and again. For example, if you were to move the laser spot in a circular
pattern first very slowly and then at ever faster speeds you would first see the small spot of light
move around in a circular motion but beyond a certain speed you would see only a circle and you
could not make out the spot any more.
The motion of the Laser spot may be produced by physically moving the Laser. This method has
some technical difficulties because the Lasers used for Laser Light Shows can be quite large
(more than 1 meter in length). Instead, the Laser beam is reflected off a mirror, and this mirror is
rotated and tilted. This can be done at very high speeds and at very high accuracies.
Professional systems are rather expensive, and even an introductory Laser Light Show system
with only one mirror costs about $500-$1000 (without the Laser). In this activity the mirror is
glued to a speaker, and the vibrations of the speaker cause the tilting and rotating of the mirror.
The light patterns produced in this way are somewhat erratic but nonetheless the principle of
Laser Light Shows is nicely demonstrated for under $50. An even less expensive version
consists of a mirror glued to a membrane (latex glove or balloon) stretched over one of the
openings of a cylindrical object (like a plastic cup with the bottom cut out). When someone
speaks into the cup, the membrane vibrates. Such a membrane is also included in the module. It
can be placed in front of the speaker by using two rubber bands. This allows for different
degrees of tension. It also provides a larger pattern than can be produced by the mirror that is
glued to the speaker.
 1997 University of Nebraska. All rights reserved.
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VI. PROCEDURE
A. SETUP
Preparation of diode laser: Before you leave check that the laser is operational. It is powered by
4 AA batteries and has a rocker switch mounted on the battery housing for on/off operation. I
recommend turning the laser off during the demo when not in use (to prolong its life but also for
safety reasons).
1. Remove the screen over the speaker.
2. Clamp laser into the “third hand”. Use the clamp that is bent open to accommodate the Laser.
3. Set up the demonstration according to the following diagrams. Aim the Laser beam at the
mirror on the speaker (or the membrane) and align both the Laser and the speaker until the Laser
spot appears at the desired location on a distant wall or screen. For safety reasons it is preferred
that the Laser points away from the audience and that the Laser beam does not move over or
through the audience. A possible arrangement is shown in the second diagram.
4. Plug he microphone or the walkman output (connect stereo cable to headset plug of walkman)
into the microphone input located on the back of the speaker.
 1997 University of Nebraska. All rights reserved.
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Laser Light Show Demo Setup
Microphone
Walkman
Back of speaker
Microphone
Speaker
Mirror
To Wall or Screen
Laser Beam
Laser
Laser
Holder
On/Off Switch
Batteries
 1997 University of Nebraska. All rights reserved.
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Laser Light Show demo arrangement in classroom
Laser Beam
Audience
 1997 University of Nebraska. All rights reserved.
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B. EXECUTION
With the walkman connected to the speaker, tune the walkman to your preferred radio station or
play a cassette tape. Adjust the volume control of both walkman and powered speaker for
distortion-free listening. This should cause the mirror on the speaker (or the membrane) to
vibrate causing the Laser beam spot on the distant wall to produce a dynamic pattern.
You can also produce nice patterns by speaking into the microphone after connecting it to the
speaker. Ask members of the audience to come forward and speak into the microphone.
C. CLEANUP
Store all hardware in their respective containers.
 1997 University of Nebraska. All rights reserved.
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VII. FREQUENTLY ASKED QUESTIONS
VIII. TROUBLESHOOTING
Problem: Laser does not work
Make sure the batteries are fully charged. Check all electrical connections. If batteries and
wiring are OK, laser needs to be examined by authorized personnel.
Problem: Laser light pattern too small or too dull
The membrane in front of the speaker may be too loose. Tighten the membrane using the two
rubber bands and try again. Try reflecting off of the mirror that is glued directly to the speaker.
Try increasing the volume or making lower tones.
IX. GLOSSARY OF TERMS
Diode Laser: One kind of Laser. A Laser produces light of a certain color that is usually very
collimated (travels in a narrow beam without spreading much) and that can sometimes have
high light intensities (see example in the safety section).
Diode Lasers are made of
materials called semiconductors (so the lasing medium is a solid; there are also liquid and
gas lasers). The ability of semiconductors to conduct electricity lies between electrical
insulators and metals. Diode lasers are very small, about 1 mm in the unpackaged form or
about 5 mm when packaged in their usual cylindrical containers. You can find diode Lasers
 1997 University of Nebraska. All rights reserved.
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in CD-ROM drives, Audio-CD players or Laser pointers. They are also used in fiber optic
communication.
Resonance: This may be described as the frequency at which a system oscillates by itself. For
example, a child's swing or the pendulum of a clock have resonant frequencies. When
pushed each swings at their own particular frequency, their resonance or eigen-frequency.
Simple systems like the swing or the clock have only one resonance. More complicated
systems like the speaker membrane may have many resonances. Another example of a
complicated system is a string of a musical string instrument like a guitar. The sound that
you hear depends greatly on the way the string is pulled or yanked.
 1997 University of Nebraska. All rights reserved.
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