Rainbow Maker
White light is really every colour of the rainbow mixed together. Shine a torch into a prism and spread the colours out on a screen.
- Light hits at
- 49.0°
- Spread of colours
- 1.58°
- Red bends by
- 38.3°
- Violet bends by
- 39.9°
The maths, live
Cauchy n = A + B ÷ λ²n(400 nm) = 1.5046 + 4200 ÷ 400² = 1.531Red lightn(700 nm) = 1.5046 + 4200 ÷ 700² = 1.513Snell 1.00 × sin θ₁ = n × sin θ₂ (violet)sin 49.0° = 1.531 × sin θ₂ → θ₂ = 29.5°Violet light slows down most in glass, so it bends most. Turn the prism to find the smallest bend (minimum deviation), then keep turning until the colours get trapped inside.
How it works
Glass slows light down, and it slows violet light a little more than red light. So glass has a slightly bigger refractive index for violet (about 1.53 in crown glass) than for red (about 1.51). Cauchy's equation, n = A + B ÷ λ², describes this.
When the light enters and leaves the prism it bends twice, following Snell's law each time. Violet bends the most and red the least, so the colours fan out into a spectrum. This spreading is called dispersion. Flint glass has more dispersion than crown glass, so its spectrum is wider.
Try this
- Which colour bends the most? Which bends the least?
- Turn the prism slowly. Find the angle where the light bends least (minimum deviation).
- Use flint glass and keep turning. When do the colours get trapped inside by total internal reflection?