White light is a mixture of colours
Dispersion is the splitting of white light into its different colours. Although sunlight looks white, it is really a mixture of seven colours. When white light passes through a prism (a triangular block of glass), the colours spread out and form a band called the spectrum.
Key idea
The seven colours in order are: red, orange, yellow, green, blue, indigo, violet. Red bends least; violet bends most.
Why the colours separate
Dispersion is really refraction that affects each colour by a different amount. Each colour travels at a slightly different speed in glass, so each is bent by a different angle. Red light bends the least and violet light bends the most. This spreads the colours apart so we can see them.
Example
A rainbow forms when sunlight passes through raindrops. Each drop acts like a tiny prism, dispersing the light into the colours of the rainbow.
Recombining the colours
The colours can be joined back into white light using a second, upside-down prism or a spinning Newton's disc painted with the seven colours. When the disc spins fast, the colours blend and it looks white — showing that white light really is made from all the colours together.
Remember
- Dispersion splits white light into 7 colours.
- A prism or raindrops can cause dispersion.
- Red bends least, violet bends most.