Refraction of Light

This lesson covers refraction of light as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand why refraction occurs, how to draw refraction ray diagrams, and — for Higher tier — how to use Snell's law to calculate refractive index.

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What Is Refraction?

Refraction is the change in direction of a wave when it passes from one medium to another. It occurs because light changes speed when it enters a material with a different optical density.

• When light passes from air into glass (a denser medium), it slows down and bends towards the normal.
• When light passes from glass into air (a less dense medium), it speeds up and bends away from the normal.
• If light hits a boundary at exactly 90° (along the normal), it passes straight through without bending — it still changes speed but does not change direction.

Key Definitions