The Anti Slit

Slit for slat


Use an overhead projector and a diffraction grating to project a "spectrum" of subtractive colors using an anti slit.



Cut a hole in the cardboard slightly smaller than the diffraction grating.
Tape the diffraction grating over the hole.

In the other piece of cardboard, cut a slit 4 to 6 mm (1/8 to 1/4 inch) wide and 10 cm long. When projected the slit should make a vertical line of light on the wall. Save the cut out strip of cardboard, this is your anti-slit. Also known as a slat.

To Do and Notice

Place the cardboard with the slit in it on the overhead projector so that the slit is projected as a vertical line on the screen. Cover the entire overhead projector with cardboard so that no light gets out except the light that comes through the slit.

Hold the cardboard-mounted diffraction grating over the exiting light from the overhead projector, rotate it until two spectra of light appear to the right and left of the slit of light. Tape it in place. In this orientation the lines on the diffraction grating will be vertical. Rotate the overhead projector or move the screen until the spectrum appears on the screen and the slit too.

Notice the spectrum of light from red through yellow then green and cyan to blue.

Take the cardboard off the overhead and replace it with the strip. Project the black strip on the screen.

Notice the strange "spectrum" of light that appears to the side of the black slit.

The colors range from cyan through magenta to yellow.

What's Going On?

Each wavelength of light in the white light from the slit is bent through a unique angle so that the wavelength we see as red light appears at one point on the screen, and the wavelengths for green and blue at other places.

The anti slit does not let light through, so at the position where red light would go from the slit, there is no red light with the anti-slit. The light at that position will be white minus red which is cyan. W -R = C. In the position of green will be white minus green or magenta,W -G = M and in the position for blue will be white minus blue or yellow. W -B = Y.

The anti-slit removes one wavelength at a time from white light. Thus we see the spectrum of subtractive colors.

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Scientific Explorations with Paul Doherty

© 1999

22 May 2000