PURPOSE: Display the spectrum of a "negative" slit.

DESCRIPTION: The white light spectrum using a slit as its source is well known. (See adjacent demonstrations.) In this demonstration, the spectrum of a "negative" slit is displayed - that is, the spectrum of a broad uninhibited white light beam with an opaque vertical rectangle in the center. How will the spectrum of this negative slit appear?

To carry out this demonstration, a three-part baffle, photographed below, is inserted in front of the light source and the light is then passed through a prism and focussed onto a screen. For comparison, the spectrum of a "normal" slit opening is shown, that of a "negative" slit the same size as the regular slit, inserted onto a wider open area, and the spectrum of a wide open area the same size as that on which the negative slit is positioned. The spectra using these three baffles are shown in the above photo, but they are washed out so that you will need to actually perform the demonstration in order to find out how it really works.

SUGGESTIONS: Ask your students to predict what they will see.


EQUIPMENT: Light source with spectrum optics and baffle with slit, wide opening, and negative slit.

SETUP TIME: 5 minutes

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