How to Make a Rainbow with a Prism

Visible or white light is made up of a range of colours each with a different wavelength. One way to see the different colours is to use a prism to split the light. When white light enters the prism it slows down and changes direction. The amount the light changes depends on the wavelength. Red light changes direction the least and violet the most.

Visible light is the part of the electromagnetic spectrum we can see. Each colour has a range of wavelengths. Red has a long wavelength and low frequency and violet has a short wavelength and high frequency.

What is a prism?

A prism is a triangular block of glass or perspex which splits light into its constituent colours.

When light enters a prism it is refracted. Each colour of the spectrum is refracted by a different amount and the colours are dispersed ( spread out ) allowing you to see them.

A prism is a great way to demonstrate visually that white light is actually made up of 7 different colours.

How to split white light with a prism

What you need to split light

Triangular prism

White cardboard

Large sheet of white paper

Dark coloured cardboard

Tape or glue

Large tray or sheet of thick card

How to use a prism

If it’s not a sunny day, you can use a torch.

Use the dark card to create a slit over a sheet of white card. Place the card so sunlight shines through giving a thin beam of light.

Place the prism over the light and rotate it until you can see the light split into the spectrum of colours.

How to split light with a triangular prism#physicsforkids — Instructions for using a prism to split light

Why does a prism split light?

White light, which enters the prism, is a mixture of different wavelengths, which get bent ( refracted ) by different amounts though the prism, allowing them to be seen separately.