Refraction through a semicircular block (including TIR) | Physics

Starter quiz

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.

A

⇔

reflected ray ✓

B

⇔

refracted ray ✓

C

⇔

incident ray ✓
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.

a

⇔

angle of reflection ✓

b

⇔

angle of incidence ✓

c

⇔

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

0° ✓

45°

90°
$The picture shows a laser beam hitting a boundary between air and a transparent solid. What is the angle of refraction?$

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

30° ✓

40°

50°

60°
The picture shows a laser beam hitting a boundary between air and a transparent solid. What is the angle of reflection?

10°

40°

50° ✓

80°
$The ray diagram shows refraction of a light wave across a boundary. If the angle of incidence changes, which of the following shows the path that the wave could follow? (original path shown in grey)$

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)

Exit quiz

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

The rays do not refract. ✓

The rays totally internally reflect.

Each ray is perpendicular to a normal line.

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

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.
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?
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).

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

0°

⇔

The beam passes through the boundary without refracting or reflecting. ✓

45°

⇔

The refracted beam travels into the air. ✓

49°

⇔

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

61°

⇔

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

Worksheet

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Lesson Details

Key learning points

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 (bigger than the critical angle) the waves will undergo total internal reflection.
The critical angle is the greatest angle of incidence before total internal reflection occurs.
For a glass–air boundary, the critical angle (c) is about 42°.

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