from Al Guenther's Science Solutions
Background: The Light Lever
Many scientific measuring instruments have dials with pointers to indicate the magnitude of the measurements. A pointer is actually a lever which magnifies the distance that the instrument moves. In ordinary clocks, speedometers and electric meters for example, the rotating shafts move very small distances, but the pointers greatly magnify these motions so they are easy to see. Unfortunately, pointers have mass which the instrument must move. In very sensitive instruments this presents a problem, as the force which the instrument measures is too weak to overcome the mass of the pointer.
The solution to the problem is a massless pointer–a beam of light referred to as a light lever! A small mirror is attached to the instrument. A beam of light is reflected off the mirror onto a screen which may be several meters distant. The beam is, in effect, a massless lever several meters long which greatly magnifies a very small movement of the instrument.
Remove both ends of a small steel can such as a soup, nut or single serving juice can. Cut a large round balloon from the mouth, over the top and back down to the mouth. Stretch one of the two halves over one end of the can and tape it securely (see diagram). For additional security it is wise to also wrap tape around the side of the can over the rubber and tape strips.
Use sticky putty (available at stationery stores for temporarily hanging posters) to fasten a small (0.5cm) mirror to the rubber midway between the center and the edge. A plastic mirror is best. The plastic mirror can be cut with a hacksaw or broken into small pieces with pliers (WEAR GOGGLES).
For a more effective oscilloscope cover the end opposite the rubber with a snap-on plastic lid. Cut a hole about 2cm in diameter in the center of the lid.
The laser and the can assembly must be attached to a holder to maintain proper alignment. To make the holder, obtain a piece of corner molding about 25cm long (from a lumber yard). Insert a screw about 10cm from one end (see diagram). Adjust the screw so that the laser beam strikes the mirror and reflects onto a wall.
When the laser and can assembly are properly adjusted, fasten them to the molding with rubber bands or tape. CAUTION: TAKE GREAT CARE TO ASSURE THAT NEITHER THE LASER BEAM NOR ITS MIRROR REFLECTION STRIKES ANYONE'S EYES.
With your mouth close to the open end of the can, make a variety of sounds of varying pitch, ranging from musical notes to raucous noise. You should observe a variety of patterns formed by the laser beam (the light lever) as it reflects from the vibrating mirror onto the shaded surface. CAUTION STUDENTS NOT TO LOOK DIRECTLY INTO THE LASER.
Things to try and notice: How is the pattern different for musical notes and nonmusical noise? Who can make the most regular geometric pattern? How does the distance from the wall affect the pattern? Try different materials in place of the balloon (like plastic wrap). What happens if objects are in the can? Try different sound sources like musical instruments.
- Small steel can
- Large round balloon
- Tape (duct tape or electrical tape is best)
- Sticky putty
- Small (0.5cm) plastic mirror
- Laser pointer
- Corner molding, 25cm
- Rubber bands