Refraction occurs when waves travel from one transparent medium to another, causing a change in direction.

A medium is the material through which a wave is travelling.

The angle of incidence is the angle between the incident ray and the normal.

Total internal reflection is when waves fully reflect from a boundary to a medium when greater wave speed, rather than being transmitted through.

The critical angle is the greatest angle of incidence before total internal reflection occurs for a boundary to a medium with greater wave speed.

Refraction through a semicircular block (including TIR)

Physics

In A-level physics, some pupils will build on this knowledge to learn about the photon model of electromagnetic radiation and the use of electromagnetic radiation for collecting evidence about the universe or for generating images of the inside of a human body.

In the Measuring waves unit, pupils learnt about the measurable features of a wave, the relationship between them and the wave equation.

Electromagnetic waves

Physical activity and equipment requiring safe use

Each ray is perpendicular to a normal line.

Each ray is travelling along a normal line.

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

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

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

Place the block back on the paper in its outline.

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

Draw crosses to show the path of the beam. 

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

Draw the normal and measure the angle of refraction.

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.

Which of the following ray diagrams shows total internal reflection?

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?

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

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

The beam passes through the boundary without refracting or reflecting.

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

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

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.

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.

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.

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

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

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

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)

Content guidance

Risk assessment required - equipment

Adult supervision required

Starter Quiz