1Splitting white light
White light is a mixture of colours. A prism refracts each colour by a different amount, spreading them into a spectrum: red bends least and violet most.
spectrum
Lesson 24.4 notes
Colour
The spectrum, frequencies, coloured objects, filters and coloured light.
2Colours and frequency
Each colour of light is a wave with a different frequency. Red light has the lowest frequency and violet the highest. Our eyes see each frequency as a colour.
frequency
3Putting the colours back
Spin a disc painted with the colours of the spectrum and it looks almost white: your eye mixes the colours back together, just as white light is made of them.
4Why objects have colours
A red apple reflects red light and absorbs the rest. A yellow banana reflects red and green, which together look yellow. White reflects every colour; black absorbs them all.
5Filters
A filter lets through only its own colour and absorbs the rest. Red light shone on a green filter is absorbed, so no light gets through.
filter
6Coloured light on coloured things
Under red light, a red shirt still looks red. Under blue light there is no red to reflect, so it looks black. A white shirt takes on the colour of the light.
7Mixing coloured light
Red, green and blue are the primary colours of light. Red and green make yellow, green and blue make cyan, red and blue make magenta, and all three make white.
primary colours
Explore: Colour lab
Switch the red, green and blue spotlights on and off. Watch the colours mix, and see how each object looks in each light.
Common mistake: Filters take colours away
A red filter doesn't dye light red. White light already contains red: the filter absorbs the other colours and lets only the red through.
Remember
- White light is a mixture of colours. A prism splits it into a spectrum: red, orange, yellow, green, blue, indigo, violet.
- Each colour of light has a different frequency: red lowest, violet highest.
- Objects reflect their own colour and absorb the rest (yellow ones reflect red and green); filters let their own colour through. Black absorbs every colour.
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