What Physics is
involved here?
Starter Question
• An optician needs to have the sight test letters
exactly 6m from the patient. Rather than
having a 6m room, describe, using the correct
scientific vocabulary, how they can use a
mirror to create the same effect.
Learning Objectives
By the end of this lesson you should be able to…
• Distinguish between refraction, reflection and the
critical angle.
• Investigate the law of refraction
• Explain that some effects seen in waves, for example
refraction, are due to the difference in velocity of the
waves in different substances.
• Draw ray diagrams and wave front diagrams to
explain refraction in terms of the change of speed.
Refraction
Light is refracted (or changes direction) when it
passes from one medium to another.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Refraction in different materials.
Watch the demonstration of the beams of light
refracted through a beaker of air, water, brine (salt
water) and glycerol – what do you notice?
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Refraction
Refraction occurs when a wave enters (or exits) a medium
with a different density to that of the medium it is in. This
speeds up / slows down the wave.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Refraction
Normal
Angle of
incidence
Angle of
refraction
Angles are measured relative to the ‘normal’ – a plane
drawn at 90° to the surface of the medium.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Investigating Refraction
Determine the amount the light refracts upon entering and
exiting a glass / Perspex block.
Follow the method carefully and get 2 tables of results one for
each material.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
SAFETY
• Be careful the ray boxes get very hot when on,
so do not touch the bulbs.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
1. Place a rectangular Perspex block on a sheet of paper and
draw around it.
2. Draw a normal at 90° to the top surface of the block.
3. Shine light rays, with angles of incidence [i] of 20°, 40, 60° and
80°, into the block at the point where the normal meets the
glass surface. Record the angle of refraction [r].
4. Repeat the investigation for rays leaving the glass block.
angle i
angle r
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Light bends _______ the normal when entering a
_________ material, this is because it _______
______ as it enters the medium.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Refraction
The speed of light depends on the material through which the
light is travelling.
When light enters a different material (e.g. from air into glass), the
speed of light changes.
This causes the light to bend or refract at the
boundary.
air
glass
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Results
Angle of
Angle of
incidence, refraction,
i ( o)
r ( o)
sin i
sin r
sin i
sin r
20
40
60
80
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Conclusion
1. What shape is the graph and what relationship does it
show?
2. Why is the experiment not very accurate?
3. Find out from your text books what Snell’s Law is and
include it in your conclusion.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Perfect results
Angle of Incidence Angle of Refraction
(degrees)
(degrees)
0.00
0.00
10.0
7.5
20.0
14.9
30.0
22.1
40.0
28.9
50.0
35.2
60.0
40.6
70.0
45.0
80.0
47.8
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
𝑆𝑖𝑛 𝑖
Graph:
Graph checklist:
Over half the paper used
Title and Axes labelled & units shown
Points plotted neatly with dot and
circle.
Line of best fit drawn going through
as many points as possible.
Gradient calculated with triangle
method and using over half of the
line of best fit.
sin 𝑟
Draw a line of best fit with a ruler and pencil, and
find the gradient of your line.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Conclusion
• The relationship between the angles of incidence and
refraction for two media is known as Snell's Law.
• Snell's law applies to the refraction of light in any situation,
regardless of what the two media are.
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Snell's Law and Refractive
Index
n= sin i
sin r
• The value ‘n’ is the refractive index.
• The greater the refractive index the greater the change in
direction seen in the light when entering the new medium.
• Some refractive indices you should know;
• Air – 1.00
• Water – 1.33
• Glass – 1.5 (depends on the type)
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.
Refractive Index
We can use the refractive index (n) to calculate the
speed of light in a medium by using the formula:
n = speed of light in vacuum
speed of light in medium
Learning Goals:
• Investigate refraction at a boundary
• Describe refraction due to change of speed • Construct ray diagrams to show refraction.
at the boundary of two media.