INTERFERENCE.

Film Loop: Interference of Waves

Length: 4:00 Min., Black and White, No Sound

An interference pattern is produced by two sources vibrating in phase. At one point the motion is frozen, and superposed marks identify the source separation and the wavelength of the periodic waves. A fixed reference mark is superposed on one of the first order maxima. Then the source separation is doubled without changing the wavelength; the mark now lies on a second order maximum in the new interference pattern. Next, keeping this separation the same, the wavelength is doubled; the fixed mark again lies on a first order maximum of the interference pattern. In the last sequence the interference pattern is slowly changed by continuously decreasing the wavelength.

APPARATUS.The water depth was about 0.8 inch, but was not critical. The periodic circular waves were produced by magnetically vibrating small spheres on the water surface; small electromagnets placed underneath the tank activated the floating spheres.

DATA:

First sequence: d = 6 cm; l = 2 cm

Double separation: d = 12 cm; l = 4 cm

Double wavelength: d = 12 cm; l = 4 cm

NOTES. The principal emphasis in the film is to show the dependence of the interference pattern on the wavelength and source separation. Other related demonstrations of interference phenomena are shown in Film-Loops 80-239 and 80-241.

In order to prevent stroboscopic effects in the projected picture the sequences were photographed with a high speed camera; the projected phenomena are slowed down by about a factor of 3.