Light - Colour: Making rainbows

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How do optical gadgets produce rainbows?

What to use

Light box, 60° triangular prism, white card screen, red, green and other coloured cellophane (or filters from a light box set).

What to do

Set up the light box with a slit to produce a single parallel-sided beam of white light.
Shine the beam into a triangular glass prism (a real one, not the triangular shape from the light box).
Aim the beam of light through the prism and onto the white screen. Rotate the prism to get the best range of colours on the screen.

Record the colours you can see.
Place a sheet of red cellophane (or red filter) in front of the light box, and observe carefully what happens on the screen.
Observe very carefully what happens to the colour of the spectrum on the screen. Do all parts of the spectrum change to red or do some parts of the spectrum disappear and other parts remain?
Try placing a sheet of dark green cellophane (or green filter) in front of the light box, and observe carefully what happens.
Try placing a sheet of dark blue cellophane (or blue filter) in front of the light box, and observe carefully what happens.

Record what happens to the image on the screen with each of the coloured filters.
Place a filter in the beam of light as it leaves the prism. What happens to the image on the screen? Try this with different filters.

Describe what happens if the coloured cellophane (or filter) is placed in the beam of light after it leaves the prism.

What is the prism doing to the white light? Try to explain your own ideas based on the evidence you have collected in these activities.