The document explains refraction of light when it travels from air into transparent materials like glass or water, emphasizing that this bending of light depends on the material and the angle of incidence. It details an experiment for measuring refraction using a rectangular block and introduces the concept of refractive index, providing calculations and study questions for further understanding. Additionally, it discusses the behavior of light rays through prisms and offers guidance for improving experimental accuracy.

1. Refraction of light
3.5

2. Refraction of light
• When a light ray travels from air into a transparent
material such as glass or water, the ray changes
direction — it is refracted.
• Refraction happens because light travels faster in air
than in other substances (Figure5.2).
• The amount by which a light ray bends when it goes
from air into another material depends on two
things:
what the material is • the angle of incidence.

3. Refraction by a rectangular block

4. Refraction by a rectangular block
1 Set up the ray box, adjusting the slit to produce a narrow ray.
2 Position the block.
3 Mark the path of the rays AB and CD by putting a series of pencil marks along their
lines.
4 Trace around the block, then remove it.
5 Use a ruler to draw the rays AB, BC, CD. Show that the rays AB and CD are parallel.
6 Use a protractor to mark the two normals.
7 Finally, measure the angle of incidence i, and the angle of refraction r. Record the
angles in a suitable table.
8 Repeat the experiment to determine six pairs of i and r. Taking it further
Devise an experiment to investigate the refraction of light by a triangular prism and a
semi-circular glass block.

5. Refractive index

6. Refractive index
The refractive index is the ratio of the speed of light in air to
the speed of light in glass.
Because n is a ratio of speeds, it has no unit.

7. Refractive index
Example. A light ray is
incident on still water at an
angle of 45° to the normal.
Calculate the angle of
refraction. The water has a
refractive index of 1.33.

8. Refraction by prisms
Notice that as the ray goes into the prism it is bent towards the
normal; as it leaves the prism it is bent away from the normal.

9. STUDY QUESTIONS
1 a) Explain what is meant by the word refraction.
b) Explain why light refracts when it travels from air into water.
a) When light travels from air into a medium such as glass or water, the light
changes direction. This is called refraction.
b) Refraction is caused by a change of speed: light slows down when it goes
into water.

10. STUDY QUESTIONS
2 When light travels from air into glass, light bends towards the normal. When light
travels from glass into air, it bends away from the normal.
a) Use this rule to show the paths of light rays through each of these glass shapes.

11. STUDY QUESTIONS
2 b) Use your knowledge of refraction, to sketcha diagram to show the path rays through
the glass of water in Figure 5.1. Use your diagram to explain why the blue and yellow
colours of the background are reverse by the refraction of the light rays.

12. STUDY QUESTIONS
3 A student uses an experimental arrangement similar to the one shown in Figure 5.3 to
measure the refractive index of a block of glass. He measures the angle of incidence as 64°
and the angle of refraction as 34°.
a) Use the student’sresults to calculate the refractive index of the glass.
n = sini/sin r
= sin 64/sin34
= 0.89/0. 56
= 1.61

13. STUDY QUESTIONS
3 b) Discuss how the studentcould improve his experiment to provide a more
accurate answer.
The experiment should be repeated several times, with different pairs of the angle of
incidence and angle of refraction, to get an average value. A graph of sin i againstsin r
could be plotted; the gradient is the refractive index.

14. STUDY QUESTIONS
4 Water has a refractive index of 1.33. A ray is incident at an angle of 30°
on to a flat water surface.
Calculate the angle of refraction for the ray entering the water.
n = sin i/sin r
1 33 = sin 30/ sin r
sin r = 0.5/1 33
r = 22o