3.5 Colour
Summary: Rainbows are beautiful patterns of light you might see after it rains. But where do the colours in a rainbow come from? Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
- describe what happens when Light passes through a prism
- understand how primary colours make secondary colours
- explain the effect of filters and coloured materials on light.
Think back
- What four things can happen when light hits a material?
- How do we see things?
- What are wavelength and frequency?
Key idea Light is a mixture of colours. We can use a prism to disperse white light into a spectrum of colours.
Key words prism, spectrum, dispersion, primary colour, secondary colour, filter
Rainbows are beautiful patterns of light you might see after it rains. But where do the colours in a rainbow come from?
How do you split white light?
White light is made up of seven different bands of light. Each one is a different colour. You can use a prism to split (divide) white light into a spectrum. This is called dispersion. The spectrum of white light is continuous. This means there are no gaps between the colours. Sir Isaac Newton first did the experiment you can see in Figure 1 in about 1666.
Figure 1A prism splits white light into a spectrum of colours. Full text alternative to the content
Dispersion happens because light with a high frequency, like violet, is refracted more than light with a lower frequency, like red.
The white light spectrum is part of a larger spectrum called the electromagnetic spectrum. Waves of this spectrum include visible light, infrared radiation, and ultraviolet radiation.
A Suggest whether the frequency of red light is higher or lower than that of green light.
What happens when you add light?
The eye can identify three colours of light, called the primary colours of light. These are red, green, and blue.
When you mix two primary colours, you get secondary colours of light. These are cyan, yellow, and magenta. Figure 2 shows that you get white light when you mix all three primary colours of light together.
B Suggest how you combine primary colours of light to make orange light.
Figure 2The Venn diagram shows the primary and secondary colours of light. Full text alternative to the content
What do filters do to light?
A red filter takes away colours from white light. It transmits red light and absorbs the rest. It does not change the colour of light. You can see how filters work in Figure 3.
C State what a blue filter does to white light.
Working scientifically A student wants to record data in an experiment. They shine all the primary and secondary colours of light onto pieces of coloured material. Draw a suitable table for their results.
Figure 3Filters transmit the colours that they are and absorb the rest. Full text alternative to the content
Why are objects different colours?
A blue car reflects blue light into the eye. When the white light from the Sun hits the car, the paint absorbs all the other colours except blue. Any coloured object reflects the colour that it is and absorbs the rest. Figure 4 shows how light reflects from red, white, and black objects.
Figure 4Red objects reflect red light, but absorb all other colours. White objects reflect all colours of light. Black objects absorb all colours of light. Full text alternative to the content
Stretch zone
- Copy and complete the sentences. When white light goes through a prism, red light is reflected/refracted the most/least and violet light is refracted the most/least. This is called dispersion/refraction. A green filter absorbs/transmits green light and absorbs/transmits the rest. A cyan object absorbs/reflects red light, absorbs/reflects blue light, and absorbs/reflects green light. A magenta object would look black in blue/green light.
- Explain why a green shirt looks black in red light.
Stretch zone
- Explain why you cannot have a white filter or a black filter.