Mirror-and-light-beam amplification turns tiny vibrations into visible beat patterns on a distant wall
Evidence
It is possible to greatly amplify the small excursions of vibrating objects by attaching a mirror to them and then shining a beam of light onto the mirror. Small differences in the angle of bounce during a vibration can be magnified into big excursions on a wall some distance away. A deflection of only 1/10 of a degree is enough to cause the spot of reflected light to move about 1 cm on a wall 5 m away from the mirror. In figure 3.13, a narrow light beam (a laser beam would be ideal today) strikes first one and then another mirror attached to its own tuning fork. The forks are slightly mistuned, so that the mirrors at first vibrate in phase, then out of phase, and so on. After bouncing off both mirrors, the light beam falls on a wall. The direction of the beam is sensitive to slight angle changes of the mirror as it rides back and forth on the tuning fork tines. If the second mirror is deftly rotated, the projection on the screen traces out the beat pattern shown in this figure. Jules Antoine Lissajous’s (figure 3.12) scheme uses this principle and is a kind of microscope, magnifying the oscillation of the vibrating prong. — Eric J. Heller, Why You Hear What You Hear- An…, 312
Categorization
- CANDY: A 1/10 degree mirror deflection moves a light spot a full centimeter 5 meters away — a neat bar-talk fact about optical lever amplification.
- XREF: The optical lever principle reappears in galvanometers and seismograph design; connects to instrumentation history.