KinexiaLEARN PHYSICS

Snell's Law — Air to Glass (Worked Example)

Medium
Problem

A ray of light in air strikes a glass block (n = 1.5) at 60° to the normal. What is the angle of refraction inside the glass?

Given

  • n₁ = 1.00 (air)

  • n₂ = 1.50 (glass)

  • θ₁ = 60°

Find

The refracted angle θ₂.

Principle

n₁sinθ₁ = n₂sinθ₂: entering a denser medium bends the ray toward the normal.

Formula

Solution

  1. Write Snell: sin⁡θ2=(n1/n2)sin⁡θ1\sin\theta_2 = (n_1/n_2)\sin\theta_1

  2. Substitute: sin⁡θ2=(1.00/1.50)sin⁡60°=0.6667×0.8660=0.5774\sin\theta_2 = (1.00/1.50)\sin60° = 0.6667 \times 0.8660 = 0.5774

  3. Take arcsin: θ2=arcsin⁡(0.5774)=35.3°\theta_2 = \arcsin(0.5774) = 35.3°

35.3° from the normal

Unit check

Degrees throughout — consistent.

What this means physically

35.3° — bent toward the normal, as expected entering glass. This bending at both surfaces is what lets lenses focus light.

Common mistake

Reporting 35.3° from the surface. Snell's angles are always measured from the normal — say so explicitly.