Refraction: how to draw a ray diagram and explain why light bends

Refraction is the change in direction of a wave when it crosses the boundary between two materials at an angle, because its speed changes. In a refraction ray diagram, you draw the boundary, a dashed normal at 90° to it, and the rays. Light entering a material where it slows down, such as from air into glass, bends towards the normal; light speeding up bends away from it.

How to draw a refraction ray diagram

1. Draw the boundary between the two materials with a ruler.
2. Where the ray meets the boundary, draw the normal: a dashed line at 90° to the boundary.
3. Draw the incident ray with an arrow pointing towards the boundary.
4. Draw the refracted ray on the other side, bending towards the normal if the light slows down, or away if it speeds up.
5. Mark the angle of incidence (i) and angle of refraction (r), both measured from the normal, not from the boundary.

This diagram is drawn to scale for an angle of incidence of 50° and glass with a refractive index of 1.5, which gives an angle of refraction of about 31°. AQA's GCSE Physics specification (higher tier) asks you to construct ray diagrams like this to illustrate refraction at the boundary between two media.

Which way does light bend?

A ray that hits the boundary along the normal still changes speed, but it doesn't change direction.

Refraction through a glass block

When light passes through a rectangular glass block, it bends towards the normal on the way in and away from the normal on the way out. Because the two faces are parallel, the ray that comes out is parallel to the ray that went in, just shifted sideways.

Why refraction happens: wave fronts

AQA's higher tier also asks you to use wave front diagrams to explain refraction in terms of the change of speed. Picture straight wave fronts reaching a glass boundary at an angle:

• One end of each wave front reaches the glass first and slows down.
• The rest of the wave front is still in air, moving faster, so it catches up.
• The wave front pivots, so its direction of travel changes, towards the normal.
• In the glass the wave fronts are closer together, because the wavelength is shorter. The frequency stays the same.

Practise using the refraction formula in our Light & Energy course, then press Mark to check your answer in the course:

The refraction practical

AQA's Required practical activity 9 (physics only) investigates the reflection of light by different surfaces and the refraction of light by different substances. A typical method for refraction:

1. Place a glass (or acrylic) block on paper and draw round it.
2. Shine a narrow ray of light from a ray box at the block and mark the incoming and outgoing rays with crosses.
3. Remove the block, join the crosses, and draw the normals where the ray enters and leaves.
4. Measure the angles of incidence and refraction with a protractor, and repeat for different angles or materials.

Use a narrow ray and sharp pencil to reduce measurement errors, and repeat each angle. For the formula linking the angles, n₁ sin θ₁ = n₂ sin θ₂, see our Snell's law guide.

Common mistakes

• Measuring angles from the boundary. Always measure from the normal.
• Bending the wrong way. Slower medium: towards the normal. Faster medium: away from it.
• Forgetting the arrows on the rays to show the direction the light travels.
• Saying the frequency changes. Speed and wavelength change; frequency stays the same.

Practise with a short timed paper

Refraction and ray diagrams are in GCSE Physics. The daily GCSE Physics Higher paper is a short timed paper of 8 questions, with a new one every day.

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Physics · Waves and lightSnell's law calculator: refraction angles and critical angleFree Snell's law calculator: find the angle of refraction, the angle of incidence, a refractive index or the critical angle, with the working shown.