INTERFERENCE.

Film Loop: Multiple Slit Diffraction

Length: 3:25 Min., Black and White, No Sound

The interference pattern of two narrow slits is shown to be similar to that produced by two point sources; the wavelengths are the same, and the slit separation equals the source separation. Using 2, 3, 4, and finally 8 narrow slits, the interference maxima are shown to become stronger directional beams; i.e. the wave fronts become straight. The zero and first order beams are emphasized by shading portions of the pattern.

APPARATUS. A long vibrating bar was used to generate the periodic straight waves. An abnormally large wave amplitude was generated so that the diffracted wave on the far side of the slits was easily visible. The water depth was about 0.8 inch. The metal barrier protruded above the water surface.

DATA AND NOTES. The angular positions of the maxima and minima for all patterns shown (2, 3, 4 and 8 slits) are the same as those of the interference pattern from two point sources which have the same wavelength and a source separation equal to the separation of the slits; first maxima at about 50 degrees and ratio l/d = 0.75. The slits were narrow enough (about half the wavelength) so that there were no diffraction nodes, but the intensity of the diffracted wave decreased with increasing angle, up to 90degrees. Therefore, the interference pattern from the multiple slits is quite weak at large angles from the normal, whereas the pattern from two point sources is strong at large angles. Even with only 8 slits in the "grating", the interference maxima are developed into very nearly non-diverging beams which head in the direction of the maxima of the double-slit pattern.

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.