Refraction of light revision notes

Edexcel International GCSE Physics (4PH1), chapter 12: Light waves, specification point 3.14, 3.16. Free lesson.

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Refraction is a change in the direction of light when it crosses from one material into another. Knowing how speed affects bending helps you draw rays through a glass block and explain when light does not bend.

How light bends at a boundary

Light changes speed when it enters a different material. This change in speed causes refraction, which is a change in direction. Angles are measured from the normal, an imaginary line at 90° to the surface. From air into glass, light slows down and bends towards the normal. Its angle in the glass is smaller than its angle in the air. From glass into air, light speeds up and bends away from the normal. Its angle in the air is bigger. Water works in the same way as glass, but the change is a little less strong. Refraction happens where light changes material. In one material,…

How to draw a ray through a rectangular block

At each surface, compare the ray's direction with the normal. Light bends towards the normal as it enters the glass, then away from the normal as it leaves. The ray emerges parallel to the incident ray, so it points in the same direction but is shifted sideways.

Why a change of speed makes light bend

A wide beam arriving at an angle does not reach the boundary all at once. One edge enters the glass first and slows while the other edge is still travelling faster in air. This makes the beam swing round towards the normal. The first edge to enter the glass slows down. The other, faster edge catches up, turning the beam. The beam ends up closer to the normal. This is why slowing down means bending towards the normal.

When light changes speed but does not bend

A ray can hit a surface along the normal. In this case, the whole beam reaches the boundary at the same time. It slows down together, so no part swings round and its direction stays the same. Along the normal, light entering glass slows down but does not change direction. A claim that this ray bends towards the normal is false.

Explaining light going from water into air

Use the speed change to work out the direction of bending. Light speeds up when it moves from water into air, so it bends away from the normal. State that the light speeds up as it enters the air. Then state that it bends away from the normal.