Sunlight Spectrum
When a white light passes through a prism, light of different colors (wavelengths) will emerge at different angles with respect to the initial beam of light.
If white light falls all over a prism, the red from one part will overlap the blue from another part, so the spectrum will be unclear.
Slit-Prism Dispersing:
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.
