Splitting white light with a prism
Shine red, green and violet light into a glass prism at different angles. Find how much each colour bends and why white light spreads into a spectrum, as in a rainbow.
Core concept
Glass has a slightly higher refractive index for violet light than for red, so violet bends most and red least; a prism therefore spreads white light into its colours (dispersion)
Topic: The human eye and the colourful world
A glass prism has two slanting faces at 60° to each other. A ray of light bends towards the normal as it goes into the glass, and bends again away from it as it comes out, so it leaves turned towards the prism's base. The total turn is the angle of deviation: deviation = i + e − 60°, where i is the angle of incidence and e the angle of emergence. White light is a mixture of colours: violet, indigo, blue, green, yellow, orange and red (VIBGYOR). Glass bends each colour by a slightly different amount: its refractive index n is about 1.51 for red, 1.52 for green and 1.53 for violet. Violet is bent the most and red the least, so the colours fan out into a spectrum. Raindrops do the same and make a rainbow.
What you will learn
- Describe the path of a ray through a glass prism.
- Calculate the angle of deviation from i and e.
- Explain dispersion: why violet bends most and red least.
- Link dispersion to the rainbow.
Before you start
- Know that light bends when it passes from air into glass (refraction).
Materials
- A glass prism, a sheet of white paper, pins and a protractor
Equipment
- A ray box or a torch with a narrow slit
Procedure
- Shine red light at 40° and record.
- Change to violet light at the same angle and record.
- With red light, find the angle that gives the smallest deviation.
- Shine red light at 70° and record.
How this simulation is simplified
- The prism is crown glass with an angle of 60°; n is 1.51 for red, 1.52 for green and 1.53 for violet light.