At a boundary to a medium where the wave speed increases, waves are refracted away from the normal.
If the angle of incidence is too big the waves will undergo total internal reflection.
For a glass–air boundary, the total internal reflection occurs when the angle of incidence is 42° or greater.

Common misconception

Pupils sometimes rote-learn what specific examples of refraction look like, rather than developing a more general understanding of the direction waves turn based on changes in wave speed.

Teach pupils the general theory of refraction and apply it to a variety of different examples where wave speed changes (e.g. sound waves refract in the opposite direction to light at an air-glass boundary due to the different wave speeds).

Keywords

Refraction - Refraction occurs when waves travel from one transparent medium to another, causing a change in direction.
Medium - A medium is the material through which a wave is travelling.
Angle of incidence - The angle of incidence is the angle between the incident ray and the normal.
Total internal reflection - Total internal reflection is when waves fully reflect from a boundary to a medium when greater wave speed, rather than being transmitted through.

Total internal reflection (TIR) can be engagingly demonstrated. Pupils can look through 45-degree triangular prisms which will act like mirrors due to TIR; a periscope can be constructed using two of these. Sending laser light through fibre optic cables demonstrates a communications application.

Teacher tip

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

The ray diagram shows what happens when a light wave meets a boundary from air to glass. Match the ray labels (A, B and C) to the names of the rays.

Correct Answer:A,reflected ray

reflected ray

Correct Answer:B,refracted ray

refracted ray

Correct Answer:C,incident ray

incident ray

Q2.

The ray diagram shows what happens when a light wave meets a boundary from air to glass. Match the angle labels (a, b and c) to the names of the angles.

Correct Answer:a,angle of reflection

angle of reflection

Correct Answer:b,angle of incidence

angle of incidence

Correct Answer:c,angle of refraction

angle of refraction

Q3.

When a ray hits a boundary between two media, it does not refract if the angle of incidence is ...

Correct answer: 0°

45°

90°

Q4.

The picture shows a laser beam hitting a boundary between air and a transparent solid. What is the angle of refraction?

Correct answer: 30°

40°

50°

60°

Q5.

The picture shows a laser beam hitting a boundary between air and a transparent solid. What is the angle of reflection?

10°

40°

Correct answer: 50°

80°

Q6.

The ray diagram shows refraction of a light wave across a boundary. If the angle of incidence changes, which of the following shows the path that the wave could follow? (original path shown in grey)

Correct Answer: An image in a quiz

Q1 Edited screenshot of a simulation by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC–BY–4.0 (https://phet.colorado.edu) Q2 Edited screenshot of a simulation by PhET Interactive Simulations, University of Colorado Boulder, licensed under CC–BY–4.0 (https://phet.colorado.edu).

Exit quiz

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6 Questions

Q1.

The diagram shows three light rays entering a semicircular transparent block. Why don’t the rays change direction when they enter the block?

Correct answer: The rays do not refract.

The rays totally internally reflect.

Each ray is perpendicular to a normal line.

Correct answer: Each ray is travelling along a normal line.

Q2.

A student wants to measure the angle of refraction (using the equipment shown) at a boundary from the block into air when the angle of incidence is 20°. Put the following steps into the correct order.

1 - Draw the outline of the block on paper then remove the block.

2 - Measure an angle of incidence 20° (using a protractor) and mark on the paper.

3 - Place the block back on the paper in its outline.

4 - Shine a beam of light so it hits the glass–to–air boundary at an angle of 20°.

5 - Draw crosses to show the path of the beam.

6 - Remove the block and draw in the path of the beam.

7 - Draw the normal and measure the angle of refraction.

Q3.

Which of the following ray diagrams shows total internal reflection?

Correct Answer: An image in a quiz

Q4.

Each diagram shows a possible path of light after it enters a semicircular glass block. In which diagram is the angle of incidence as large as possible without total internal reflection occurring?

Correct Answer: An image in a quiz

Q5.

Total internal reflection can happen …

at any boundary between two media.

at a boundary where the ray refracts towards the normal (e.g. air to glass).

Correct answer: at a boundary where the ray refracts away from the normal (e.g. glass to air).

Q6.

For light travelling from water to air, the largest angle of incidence possible without total internal reflection is 49°. Match each angle of incidence to the outcome.

Correct Answer:0°,The beam passes through the boundary without refracting or reflecting.

The beam passes through the boundary without refracting or reflecting.

Correct Answer:45°,The refracted beam travels into the air.

The refracted beam travels into the air.

Correct Answer:49°,The refracted beam travels along the boundary between water and air.

The refracted beam travels along the boundary between water and air.

Correct Answer:61°,The beam is totally internally reflected (back into the water).

The beam is totally internally reflected (back into the water).

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