Light and Sound » Practical: Investigating Refraction

What the practical is for

This practical is named in the specification, so it can be examined. You do not need to have done it in a laboratory, but you must be able to describe the method, the results and the problems as if you had.

The aim is to see how light is refracted by three shapes of glass (or clear plastic) block. Remember, light bends towards the normal going into glass and away from the normal coming back out into air.

Apparatus: a ray box with a single slit, a power supply, a rectangular block, a semi-circular block, a triangular prism, plain white paper, a sharp pencil, a ruler and a protractor.

Key terms:

• Ray box: a box with a lamp inside. Light can only get out through the front.
• Slit: a narrow gap in a card or plate on the front of the ray box. It lets out one thin ray of light that you can see on the paper.
• Semi-circular block: a block shaped like half a circle (a letter D), with one flat face and one curved face.
• Triangular prism: a block whose top is a triangle, so its faces slope towards each other.
• Spectrum: the band of colours, red to violet, that white light spreads into.

The method: tracing a ray

You cannot draw on a beam of light, so you mark where it goes and join the marks afterwards.

1. Place the block in the middle of a sheet of plain paper. Draw round the block with a sharp pencil.
2. Dim the room and switch on the ray box. Aim the ray at one face of the block, at an angle.
3. Mark the incident ray with two crosses, one near the ray box and one near the block. Put each cross on the centre of the ray.
4. Mark the emergent ray, where the light comes out of the block, with two more crosses. Again, keep them far apart.
5. Switch off the ray box and remove the block.
6. With a ruler, join the first two crosses and carry the line on to the outline of the block. Do the same for the emergent ray. Then join the point where the light went in to the point where it came out. This line is the path of the ray inside the block.
7. Draw the normals. Use the protractor to draw a dashed line at 90° to the face at the point where the ray goes in, and another where it comes out.
8. Measure the angle of incidence and the angle of refraction with the protractor. Both are measured from the normal.
9. Put the block back on its outline and repeat for other angles of incidence.

Two crosses far apart give a more accurate line than two crosses close together, because a small slip in one cross changes the direction of the line less.

What you see with each shape

Why the curved face does not bend the ray. A ray that enters at the centre of the flat face travels through the glass along a radius of the half circle. A radius always meets the curve at 90°, so the ray meets the curved face along the normal. A ray along the normal does not change direction. So all the bending happens at one point, which makes the angles easy to measure.

The spectrum from a prism. Hold a white card where the light leaves the prism. Instead of a white patch you see a band of colours. Red light is bent the least and violet light is bent the most, so the colours are spread out with red at one edge and violet at the other.

Results

For the rectangular or semi-circular block, record the angles in a table. Typical results for a glass block look like this.

Angle of incidence in °	Angle of refraction in °
15	10
30	19
45	28
60	35
75	40

The conclusion: going from air into glass, the angle of refraction is always smaller than the angle of incidence, and it increases as the angle of incidence increases. Notice that it does not double when the angle of incidence doubles (30° gives 19°, but 60° gives 35°), so the two angles are not directly proportional.

The independent variable is the angle of incidence, the dependent variable is the angle of refraction, and the block is kept the same.

Errors and how to reduce them

Problem	What to do
The ray is thick, so it is hard to tell where its path is	Use a narrower slit and mark the centre of the ray each time
Reading the protractor wrongly	Put the centre of the protractor exactly on the point where the ray meets the face, line up its zero line with the normal, and read the correct scale. A protractor reads to the nearest 1°
The block gets knocked	Check it is still on its outline before marking each ray

Repeat each reading and check that the results agree. A result that does not fit the pattern should be measured again.

Safety

Exam-style question

A student investigates refraction using a ray box and a semi-circular glass block. She aims a ray of light at the centre of the flat face of the block.

(a) Describe how the student should find the angle of refraction for this ray. [4 marks]

(b) Explain why the ray does not change direction as it leaves the block through the curved face. [2 marks]

(c) The ray from the ray box is 4 mm wide. Suggest one way the student could improve the accuracy of her measurements. [1 mark]

(d) State one safety precaution the student should take, and give a reason for it. [2 marks]

(e) The student replaces the block with a triangular prism and shines white light at it. State two things she would observe about the light leaving the prism. [2 marks]