Refraction · Grade 10
Light changes speed when it passes from one material into another, and unless it hits along the normal, this makes it change direction – refraction. It is why pools look shallower than they are and straws look bent. Snell’s law predicts the angles, and above the critical angle light is totally internally reflected, which is how optical fibres carry the internet.
Key facts
- Entering a denser material (air → glass), light slows and bends towards the normal; leaving, it bends away.
- Refractive index n = c ÷ v (c = 3.0 × 10⁸ m/s); water 1.33, glass about 1.5, diamond 2.42.
- Snell’s law: n = sin i ÷ sin r (from air), or n₁ sin θ₁ = n₂ sin θ₂.
- Critical angle: sin c = 1 ÷ n (into air), or sin c = n₂ ÷ n₁.
- Total internal reflection: from denser to less dense, with angle of incidence greater than c.
- Frequency stays the same; wavelength in the material is λ ÷ n. Apparent depth = real depth ÷ n.