PURPOSE: To show white light fringes using a Michelson interferometer.

DESCRIPTION: This experiment uses the laser and white light combination Michelson interferometer setup, which remains set up and aligned and only needs the light source to be installed. The system is pre-aligned according to procedures in the file. 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 below show some of the color patterns using this interferometer.

You can see a series of videos of the mirror moving outward (available in Real Media, QuickTime and Windows Media.) or of the mirror moving inward (avialable in Real Media, QuickTime and Windows Media.)

Also available is a (very long) video (in Real Media, QuickTime and Windows Media.) which shows the mirror moving through the entire coherence region for the white light interferometer, that is, the entire region of path length difference over which the interference is sufficiently coherent as to allow color to be seen and identified. Beyond these mirror position limits, the interference is so complicated thatit is impossible to see any coloration with the naked eye, although a spectrometer would show the exact spectrum.

A neat finale to the experiment is to burn a match, blow it out and stick the hot head into one path of the interferometer, producing heat fringes.

SUGGESTIONS: This setup is sensitive to mirror position; the correct position is written on the micrometer screw adjuster for the moveable mirror. Check that you have it aligned before trying to demonstrate it.

See Question of the Week #225 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 6D40.10) See Demonstration Reference File for alignment procedure using a laser as well as a large number of articles dealing with thenwhite light Michelson interferometer.

EQUIPMENT: Michelson interferometer, bright white light point source with condenser lens and heat filter, pre-set as photographed.

SETUP TIME: 5 min.