Background

Collimated light waves come from the laser and pass through a pair of narrow slits in the slide; the light passes through and then projects on the distant screen. But light travels as an electromagnetic wave, so when the light comes out of the two slits, it forms two wavefronts, just like ripples from two stones dropped in a pond. These two wavefronts can interfere with each other, as we can model with this pair of overlapping concentric circles. Where two peaks or two valleys of the wave pattern line up, they add together, interfering constructively; when a peak and a valley overlap, they cancel out, interfering destructively. The same happens with light waves; the light from the two slits overlaps, and creates a pattern of bright spots (constructive interference) and dark spots (destructive interference). The spacing between the bright and dark fringes ultimately depends on three things: the distance between the slits and the screen, the wavelength of the light, and the spacing between the two slits.

Two identical transmitting antennas for microwaves (shiny strips mounted on meter stick) are driven by the same source, creating two coherent sources. Scanning the region in front of the two sources reveals an interference pattern, which can be seen by a class using a projection meter connected to the microwave receiver.

The photograph at the left shows the receiving antenna at the central maximum; at the right the receiving antenna is at the first minimum.

This experiment uses the laser and white light combination Michelson interferometer setup. Because alignment requires a laser, this demonstration will be delivered (and can be used) with a laser installed. White light from a bright point source is collimated by a condenser lens and passes through a heat filter directly into the interferometer. The light exiting the interferometer is focused onto a distant screen, providing a field about one foot in diameter, clearly visible over the entire lecture hall. The fringe colors can be seen to be negative colors, that is, complementary colors to the colors to the spectral colors which are eliminated by destructive interference.

The photographs above show some of the color patterns using this interferometer.

This demonstration is very sensitive to alignment and temperature, and is not recommended for routine classroom use.

This demonstration is very sensitive to alignment and temperature, and is not recommended for routine classroom use.