DOPPLER EFFECT.

Film Loop: The Doppler Effect

Length: 3:45 Min., Black and White, No Sound

A pulsed air jet producing a periodic circular wave first moves over the water surface at about 1/3 of the wave velocity. The Doppler effect is clearly seen . At one point the motion is frozen on the screen to permit close examination of the wavelength differences. The source is also shown moving at twice the previous velocity.

APPARATUS. The water depth was not critical. The wave generator was a small drum hit by a vibrating clapper mounted on a cart which moved uniformly along the edge of the tank. A narrow tube from the drum protruded out over the tank and directed puffs of air onto the water surface.

NOTES. It took only 10 sec for the source to move across the tank at 1/3 times the wave velocity. In order to prevent stoboscopic effects and to be able to observe the effect for a reasonable time the sequence was photographed with a high speed camera. The film is designed to be screened at 16 frames per second (silent speed); the projected phenomena are slowed down by about a factor of 6. The magnitude of the Doppler effect shown here, with a ratio of source to wave velocity of about 1/3 to 2/3, is large compared to what we normally hear or record. The ratio is then usually 1/10 or 1/20; e.g. when we hear the change of pitch of a car horn or a train whistle as it moves past us at 30 to 60 mph. When the ratio of source to wave velocity is greater than 1, a shock wave occurs. (see Film 80-238) In Fig. 1 (not included here) a stationary observer in front of the source (on the right) sees athe source approaching and measures a higher than normal frequency (pitch); an aobserver behind the source (on the left) sees the source receding and measures a lower than normal frequency.