Refraction in four steps: what really happens when light enters glass

Put a pencil in a glass of water and it seems to snap at the surface. Nothing happened to the pencil. What changed is the path of the light coming from it. That bending of light at the boundary between two materials is called refraction, and it appears in Class 10 Science in the chapter “Light – Reflection and Refraction”.

Here is the whole idea in four steps — the same four steps the refraction animation in the app walks through.

1. Arrive at an angle

A ray of light travels through air and meets a flat glass surface. We measure its direction from the normal — an imaginary line at 90° to the surface. Say the ray arrives at 45° to the normal. That is the angle of incidence.

2. Cross the boundary

Glass is optically denser than air, so light travels more slowly in it. One thing does not change as the ray crosses: its frequency. The frequency is set by the source of the light, so it stays the same on both sides of the boundary.

3. Apply Snell’s law

Take air as n₁ = 1.00 and glass as n₂ = 1.50. With θ₁ = 45°: sin θ₂ = (1.00 × sin 45°) / 1.50 ≈ 0.471, so θ₂ ≈ 28.1°. The ray bends towards the normal.

4. Travel through the glass

Inside the glass the light is slower, and because its frequency hasn’t changed, its wavelength is shorter. When it leaves the glass back into air, the same rule works in reverse and the ray bends away from the normal.

Check yourself

• Which quantity stays the same when light passes from air into glass? (Frequency.)
• If glass had a higher refractive index, would the ray bend more or less? (More — θ₂ would be smaller.)
• Why does a pool look shallower than it is? (Light from the bottom bends away from the normal as it leaves the water.)