Lesson 1 of 4 · Form 3 Physics
Refraction of light
18 min
What is refraction?
The bending of light as it passes from one medium to another because its speed changes.
- Into a denser medium (air → glass): bends towards the normal.
- Into a less dense medium: bends away from the normal.
Snell's law
n = sin i ÷ sin r (light from air into the medium)
Also n = speed in air ÷ speed in medium = real depth ÷ apparent depth.
Example: i = 60°, r = 35°. n = 0.866 ÷ 0.574 ≈ 1.51.
Critical angle and total internal reflection
When light goes from dense to less dense and the angle of incidence exceeds the critical angle c, all light is reflected.
sin c = 1 ÷ n. For glass (n = 1.5), c ≈ 42°.
Applications
Optical fibres, prism periscopes and binoculars, mirages, the sparkle of diamonds.
Dispersion
White light splits into colours through a prism: ROYGBIV. Violet bends most, red least.
Key points to remember
- n = sin i / sin r
- sin c = 1/n
- TIR needs dense to less dense and i > c
- Violet bends most in dispersion
Test yourself
