1. Breaking up white light
10.4 Dispersing light
Breaking up white light
You’ve just seen how a prism can break up white light into 7 colours.
This breaking up of white light is called dispersion. The prism disperses the light.

2. RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET 10.4 Dispersing light
Colours
When white light is broken up like this the 7 colours are always in the same order.
RED
ORANGE
YELLOW
GREEN
BLUE
INDIGO
VIOLET
top
bottom

3. 10.4 Dispersing light Colours How can you remember the order?
There’s the famous spy:
ROY GBIV
Or a defeated British king:
Richard Of York Gained Battle In Vain
See if you can think of your own.

4. 10.4 Dispersing light
Colours
You can see the order in this rainbow.

5. Why does light disperse like this?
10.4 Dispersing light
Why does light disperse like this?
Remember that light is a wave.
wavelength
The size of a wave is measured from one peak to the next and is called its wavelength.

6. Why does light disperse like this?
10.4 Dispersing light
Why does light disperse like this?
White light is made up of all the colours of the rainbow.
Each colour has its own wavelength.
Red has the longest and violet has the shortest wavelength.

7. Why does light disperse like this?
10.4 Dispersing light
Why does light disperse like this?
When light enters a glass prism it slows down.
But the shorter wavelengths slow down most and so bend more sharply.
Blue is a short wavelength.
Red is the longest one.

8. Why does light disperse like this?
10.4 Dispersing light
Why does light disperse like this?
As each wavelength, or colour, slows down at a different rate it therefore has a different angle of refraction to the rest.
This means the different colours spread out.
REMEMBER! Blue bends best.

9. Why does light disperse like this?
10.4 Dispersing light
Why does light disperse like this?
The shape of the prism means they spread out even more on leaving the prism.
You may see a small spectrum with a rectangular glass block but the colours do not spread out as much.

10. 10.4 Dispersing light Recombining colours
When a second prism is added ‘upside down’ the dispersion of the second prism is in the opposite direction to the original dispersion.
White light is formed again.

11. 10.4 Dispersing light Recombining colours
Look carefully. The two triangular prisms form a parallel-sided glass block.
We didn’t see dispersion when we passed a ray of light through a rectangular glass block.

12. 10.4 Dispersing light Spinning disc
The image stays on the retina at the back of the eye for about 1/10 of a second.
When the disc spins very fast all the colours of the rainbow appear on the retina together.
The colours all combine to form white light.

13. 10.4 Dispersing light How do you see a rainbow?
A rainbow is formed by a combination of dispersion and total internal reflection.
Raindrops act like a prism for the light of the sun.
This means that you only see a rainbow when the sun is behind you and hitting raindrops at a certain angle.

14. 10.4 Dispersing light
How do you see a rainbow?