Physics at the Party: Dancing Lasers

Add some light to your night. Make your party one that your friends remember, let those lasers dance!

What you Need

• 1 laser
• A speaker or subwoofer (subwoofer works best)
• Glue gun
• Glue sticks
• Spandex
• Tape
• Large cup
• Scissors
• 1 small mirror, approximately 2 cm x 2 cm (available at any craft store glass shop)

What to Do

1. Secure the spandex over the speaker, so that it covers the front side of the speaker completely (as shown in figure 1).
   
   

   Figure 1

2. Attach mirrors to the spandex on the front side of the speaker with a dollop of hot glue.  Be careful to put the mirrors near the center, away from the rim of the speaker and don't touch the hot glue or the tip of the hot glue gun.
3. Tape the "on" button down so that the laser pointer is always on.  Be careful not to shine it in your or anybody else's eye
4. Poke a hole in the side of your cup toward the bottom. Poke your laser through the hole so that it faces outward and turn the cup upside down, making a stand for you laser.
5. Shine the laser toward a mirror on the speaker.  Once you have the laser lined up and shining on the mirror, tape the laser pointer in place so that it is always shining on the mirror.
6. Locate the reflection of the laser and once you've located it, turn on your ipod of CD player and let the music play.
7. Watch the laser and look at the patterns that are made.

What's Going On?

Figure 2

When a laser is shined on a mirror the mirror reflects, bounces, the light beam back. The angle that the laser enters the mirror is called the incident angle and the angle that the light exits the mirror is called the reflected angle. When the incident angle changes or the angle of the mirror changes, it effects the angle of reflection. To further understand this, stand in front of a mirror and point a laser pointer at it. If you point the laser pointer at a downward angle, then the reflected laser point may end up on the floor. If you do the same experiment but this time hold the laser by your hip and shine it at the mirror at an upward angle, then the reflected laser point will probably be at or near the ceiling. If you want to get technical you may also notice that whatever angle the light hits the mirror, it will reflect back at the same angle from an imaginary line coming straight out of the mirror, called the normal. When the speakers vibrate from the sound coming out of them, the mirror vibrates and changes the normal; therefore, changing the incident angle which then changes the reflected angle. This happens with every vibration and since light travels quickly, we can see the changing vibration by watching the patterns that the laser casts on the wall opposite the mirror.

Try This!

• Try tilting the laser at different angles. What effect does it have on the dancing laser show?
• Try the experiment with different music how does the amount of bass in each song effect the dancing lasers?
• Try the experiment with a flashlight instead of a laser. How do the effects differ?