Topic 13.1
Refraction of light
13.1 Refraction
Aims
In this experiment you will measure the refractive index of a glass or a perspex block. This will enable you
to understand the laws of refraction for a boundary between two different substances of different
refractive indices.
Safety
 The bulbs get hot so care should be taken.
 Check all wires for loose leads/connections.
 This experiment will be conducted in a darkened room. You should therefore ensure that bags and other
objects are tucked away or put in cupboards so that nobody trips over them.
Equipment, materials and method
1 For this experiment you will require some white paper, a sharp pencil, a glass or perspex block, a ray box
with a focusing lens, a protractor and a ruler.
2 On a blank piece of paper use a ruler to draw a line, approximately parallel to one side, about a third of
the way along the paper. Leave the ruler in place.
3 Carefully position the glass block along the ruler without allowing the ruler to move.
4 Now hold the glass block in place and carefully remove the ruler. Place the ruler against the other side of
the glass block and remove the block.
5 Draw a line along the ruler. You have now precisely marked the edges of the glass block.
6 Reposition the block between the two lines you have drawn.
7 Send a ray of light into one side of the block at an angle. Mark its path into and out of the block with
small crosses at the centre of the rays.
8 Remove the block and draw in the rays. Mark the normals.
9 Measure the angles of incidence and refraction at the point where the ray enters the block. It may be
necessary to extrapolate the lines representing the rays.
10 Repeat several times for a range of angles of incidence.
Questions
1 What happens to the velocity of the light as it enters the block?
2 Does the light bend towards or away from the normal?
Results
Record your results in a table. Calculate sin i and sin r for each set of readings. Plot a graph of sin r against
sin i. Measure the gradient of your graph and from this determine a value for the refractive index.
Discussion
How can you tell from your graph whether the block obeys the rule n =
AQA Physics A AS Level © Nelson Thornes Ltd 2008
sin i
?
sin r
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Topic 13.1
Refraction of light
Refraction
Aims
 To measure the refractive index of a glass or a perspex block.
 To understand the laws of refraction for a boundary between two different substances of different
refractive indices.
 To follow a set of written instructions carefully and understand the scientific terms used.
Safety
 The bulbs get hot so care should be taken. Check all wires for loose leads/connections.
 Before the room is blacked out, ensure that students move all belongings under desks or into cupboards
and that stools and other furniture are not obstructing walkways.
 CLEAPSS information on ray boxes: Laboratory Handbook/CD-ROM section 12.16.
Teaching notes
3 This experiment should take about 20 minutes.
4 Encourage students not to press hard with pencils. A sharp pencil with a light touch makes the most
precise markings.
5 Emphasise the importance of precision when drawing lines, placing the block and positioning the ray.
Answers to questions
6 The light ray slows down.
7 The ray bends towards the normal.
Discussion
If the points lie in a straight line it obeys the rule, allowing for uncertainties in the measuring.
Practical assessment opportunities
8 PSA – Selecting and using equipment: 0–3 marks
(Following instructions and group work; organisation and safety assessed at end of course.)
9 ISA preparation:
practical task – practice in making precise and reliable measurements
written test – carrying out calculations
AQA Physics A AS Level © Nelson Thornes Ltd 2008
2
Topic 13.1
Refraction of light
Refraction
Equipment and materials required
For this experiment each student or pair of students will require the following equipment:
 six sheets of white paper
 a sharp pencil
 a protractor
 a ruler
 a glass or perspex block
 a ray box with a focusing lens (the lens may not be necessary if the slit used is sufficiently narrow) and
associated power supply
 graph paper
CLEAPSS information on ray boxes: Laboratory Handbook/CD-ROM section 12.16.
Safety
 The bulbs get hot so care should be taken.
 Check all wires for loose leads/connections.
 Before the room is blacked out, ensure that students move all belongings under desks or into cupboards
and that stools and other furniture are not obstructing walkways.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
3