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. What will the spectrum of this negative slit look like?
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 focused 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.
Below the photograph of the baffle used in the demonstration are three spectra, left to right: the standard white light spectrum, the spectrum of the negative slit combined with the regular white light spectrum, and the complete spectrum of the three parts of the baffle shown immediately below. (Note that the parts of the spectra are inverted with respect to the baffle due to the lens that focuses the image of the slit onto the screen.) The negative slit removes the spectral colors from the continuum created by the open baffle, producing a sequence of "negative" colors (right to left): cyan (minus red), blue (minus yellow), magenta (minus green), red (minus cyan), and yellow (minus blue).