What it shows: A room-size laser interferometer with audio signal output. A standing wave is produced whenever a wave is reflected back on itself. A resonant cavity requires a second reflection so that the twice reflected wave has the opportunity to be in phase with the original wave. Here, laser light is reflected from a half-silvered mirror (mounted on a wall) so as to return to the laser and be reflected again by the laser. Movement of the wall by half a wavelength is sufficient to change the cavity formed between laser mirror and wall mirror from one resonant length to another, causing the voltage from the photo detector to go from maximum to minimum and back again.
How it works:
Laser light is transmitted onto a solar cell. Directly in front of the solar cell is a half silvered mirror that allows some of the original light to reach the cell; the remaining light gets reflected between the mirror and the partially reflective front surface of the laser in such a way as to produce interference at the cell surface. Tiny changes in distance between the laser and the detector changes the interference condition at the solar cell, causing the detector to produce a voltage signal that gets converted to audio output. These changes in distance can result from floor vibrations in the vicinity of the laser or from pushing the sturdy door frame where the detector is mounted.
Let L be the distance from the laser to the detector mirror, and let l be the distance from the detector mirror to the solar cell. The total distance traveled by the light from the laser directly to the cell is
L + l.
The total distance traveled by the light that gets reflected between the mirror and the reflective front surface of the laser is
3L + l.
3L + l - (L + l) = 2L
is the difference in distance traveled by the two beams; this difference may result in total constructive interference (when 2L is an integer multiple of laser wavelengths), total destructive interference (when 2L is an integer multiple of laser wavelengths plus one half wavelength), or some combination thereof (for all other values of 2L).
Now suppose we change the distance between the laser and the detector by less than half a wavelength, call this distance d. So
3(L + d) + l - (L + d + l) = 2L + 2d
is the new difference in distance traveled by the light, which will result in a different interference condition at the solar cell. This different interference condition results in a change in intensity at the solar cell, and this change results in an audio signal.
The frequency of the audio signal is proportional to the rate of change of distance and the duration of the signal at a given frequency is proportional to the total change in distance. For example, suppose you hear an audio signal frequency of 1 kHz lasting about half a second, then 500 maxima were detected by the solar cell. In a standing wave, the maxima occur every 1/2 wavelength, so the wall move 250 wavelengths. HeNe wavelength is 632.8 nm, so the wall moved about 0.16 mm.