A Slit

1. A narrow cut or opening; a slot.
2. (vulgar, slang) The opening of the vagina.
3. (vulgar, slang, derogatory) A woman, usually a sexually loose woman; a prostitute.

If the white light passes through a slit before it strikes the prism, and then the spread-out light falls on a screen.

On the screen, will be a succession of images of the slit in different colors. At each position on the screen the image of the slit will be at a particular wavelength. Missing colors from the white light, will show up as a gap on the screen in the shape of the slit.