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The information below is intended to provide a description of the demonstration, an explanation for elementary students, and further explanation for high school students.

Please keep in mind that not all demonstrations are presented at each show.


LASER LIGHT SHOW

Figure I. Figure II. Figure III.

Description

Equipment: Several lasers of different colored light
Several small mirrors -- these mirrors are glued to a neoprene sheet and stretched over a small speaker box
Another set of small mirrors
Boom box

Step 1: The lasers are turned on and aligned to strike the mirrors. The second set of mirrors are arranged so that the laser beams reflect off both sets of mirrors to shine on a nearby wall. (The setup is displayed in Figure I.)

Step 2: The general properties of lasers are discussed and blackboard erasers are clapped, producing chalkdust in the air. The laser beams can then be seen in the air. (See Figure II.) (Sometimes the "steam" from the Liquid Nitrogen is used instead of chalkdust, as displayed in Figure III.)

Step 3: Finally, the boom box is turned on and music is played. The dots of the laser endpoints on the wall bounce and move in rhythm with the music. The light apparently "dances" to the tune of the music, forming circles and doodles which move to the music.

Description

Basic Ideas: Unlike normal light, which spreads in all directions, lasers are constructed so that a laser beam travels only in a straight line. A laser beam travels in a straight line until it collides with an object, then spreading in all directions like normal light.

Sound is caused by air particles vibrating back and forth very rapidly. These air particles collide with their neighboring air particles and spread throughout space.

When the human eye is exposed to an image, this image is retained for a very brief period of time, even if the image is no longer there. This effect is called persistence of vision.

Step 1: Once the setup is complete, the laser beams reflect off the mirror and shine through the air until they hit the wall. Since the light is not reflecting off anything until it hits the wall, you can not see the laser beam except for the small dots on the wall.

Step 2: When the chalkdust is added to the air, the laser beams reflect off the tiny particles of chalk which are in the air. The chalk dust particles reflect the laser beam, spreading the light in all directions. The laser beam can therefore be seen.

Step 3: Sound causes the air molecules to bounce back and forth rapidly. These air molecules collide with the neoprene sheet which is holding the mirrors. Since neoprene is very flexible, the air molecules cause the neoprene to jiggle. The jiggling neoprene causes the mirrors to move and the laser beams are therefore reflected at a slightly different angle. This change of angle causes the dots which appear on the wall to move. The dots are moving very quickly, and although at any given time, there is only one dot per laser displayed on the wall, it appears as if there are circles and doodles and lines. The apparent shapes which are seen are due to persistence of vision.

Description

Basic Ideas: Unlike normal light, which spreads in all directions, lasers are constructed so that a laser beam travels only in a straight line. A laser beam travels in a straight line until it collides with a molecule which is large enough to interact with the light. Once the laser beam has collided with a large enough particle, it will reflect and spread in all directions like normal light.

Sound is longitudinal waves through air.

When the human eye is exposed to an image, this image is retained for a very brief period of time, even if the image is no longer there. This effect is called persistence of vision.

Step 1: When the setup is complete, the laser beams strike the mirrors and reflect onto the nearby wall. Since the laser beam does not collide with anything until it reaches the wall, the only evidence of the laser beams is the dots on the wall.

Step 2: When the chalkdust is added to the air, there are particles of chalk in the air. The molecules of chalkdust, although small, are large enough to interact with the light of the laser beams. When the light of the laser beams collide with these particles, they shine in all directions. Since the light reflects in all directions, you are able to see the light. (One of the directions is straight toward your eyes!)

Step 3: The sound caused by the small stereo creates longitudinal waves of air. When these waves reach the neoprene with the mirrors attached, the air molecules cause not only other air molecules to vibrate, but they cause the neoprene to vibrate as well. This is due to the fact that neoprene is a very pliable solid. Since the sound waves cause the neoprene to vibrate, the neoprene moves in various directions. The vibrating neoprene moves the mirrors, causing the laser beams to be reflected in a slightly different direction. This change in reflection will correspond to the dots moving around on the wall. Due to the rhythmic nature of music, the dots will move in a more orderly fashion than complete randomness, creating the exciting effect of circles and doodles and lines. The reason that one sees the shapes rather than simply the dots moving in a circular fashion is the persistence of vision. The dots are moving so fast that the human eye perceives them all at the same time, even though at any given time, there is only one dot per laser shining on the wall.

Related Topics

The following physics topics are discussed during this demonstration:

Sponsored by the Physics Department and the Center for Science, Mathematics, and Engineering Education -- University of Virginia