KS4 Physics
Refraction
1 of 20
39
© Boardworks Ltd 2005
2004
Contents
Refraction
Refraction
Refractive index
Refraction effects
Lenses
Summary activities
1
2 of 20
39
© Boardworks Ltd 2005
2004
Refraction in water waves
When waves in water travel through water of different depths
they change speed. In shallow water the waves slow down; in
deeper water they speed up.
We can investigate this by changing the depth of the water in
a ripple tank.
As the water waves slow
down, their direction
changes due to the
change of speed. This is
called refraction.
Perspex sheet used to
change depth of water
1
3 of 20
39
© Boardworks Ltd 2005
2004
Why does refraction happen?
Imagine a car driving from the road into a muddy field.
 In the muddy field it slows
down as there is more friction.
 If it enters the field at an angle
then the front tyres hit the mud
at different times.
 Tyre 1 hits the mud first and
will move more slowly than
tyre 2. This causes the car to
turn towards the normal.
 When the car leaves the mud
for the road, tyre 1 hits the
road before tyre 2 and this
causes the car to turn away
from the normal.
1
4 of 20
39
road
tyre 1
tyre 2
mud
© Boardworks Ltd 2005
2004
Why does refraction happen?
If the car approached the muddy field at an angle of
incidence of 0° then both front tyres would hit the mud at
the same time.
The tyres would have the same speed relative to each
other so the direction of the car would not change, it
would just slow down.
1
5 of 20
39
© Boardworks Ltd 2005
2004
Refraction
The speed of light waves depends on the material they
are travelling through.
air = fastest
glass = slower
diamond = slowest
If light waves enter a different material (e.g. travelling
from glass into air) the speed changes.
This causes the light to bend or refract.
air
glass
1
6 of 20
39
© Boardworks Ltd 2005
2004
Refraction at an air-glass boundary
1
7 of 20
39
© Boardworks Ltd 2005
2004
What happens in refraction: air to glass
When light is refracted as it travels from air to glass:
angle of incidence > angle of refraction
i > r
As the light ray travels from
air into glass it moves
towards the normal.
In general, when light rays
move from a less dense
medium (air) to a more dense
medium (glass) they ‘bend’
towards the normal.
1
8 of 20
39
i > r
air
glass
© Boardworks Ltd 2005
2004
Refraction through a glass box
What happens when a light ray passes from glass into air?
1
9 of 20
39
© Boardworks Ltd 2005
2004
What happens in refraction: glass to air
When light is refracted as it travels from air to glass:
angle of incidence < angle of refraction
i < r
As the light ray travels from
glass into air it moves away
from the normal.
In general, when light rays travel
from a more dense medium (glass)
to a less dense medium (air) they
‘bend’ away from the normal.
If the two surfaces of the block are
parallel, then the ray at the start is
parallel to the ray at the end.
1
10ofof20
39
glass
air
i
<
r
© Boardworks Ltd 2005
2004
Refraction – angle of incidence
What happens to light travelling from air through a glass
block when the angle of incidence is 0°?
 i = 0°
When the angle of
incidence is 0 the light
ray is not deviated from
its path.
air
glass
undeviated light ray
1
11ofof20
39
© Boardworks Ltd 2005
2004
Contents
Refraction
Refraction
Refractive index
Refraction effects
Lenses
Summary activities
1
12ofof20
39
© Boardworks Ltd 2005
2004
Travelling through different materials
If you were running along a beach and then ran into the
water when would you be moving slower – in the water
or on the sand?
In the water.
In a similar way, as light moves from one medium to
another of different density, the speed of light changes.
Do you think light moves faster or slower in a more dense
medium?
Light moves slower through a more dense medium.
1
13ofof20
39
© Boardworks Ltd 2005
2004
The speed of light
Glass
From this bar chart, which
material do you think is denser,
glass or water?
Speed of
light
(thousands
km/s)
Water
As light enters denser
media, the speed of
light decreases.
300
270
240
210
180
150
120
90
60
30
0
Vacuum
Light travels at
300,000 km/s in a
vacuum.
Glass must be denser than water because light travels
more slowly through glass than water.
1
14ofof20
39
© Boardworks Ltd 2005
2004
The speed of light
We can study refraction of light by comparing its speed in
air to that in a different material.
A number called the refractive index is the ratio of
these two speeds:
Refractive index
=
speed of light in air
speed of air in material
Example:
The speed of light in air is 300,000,000 m/s, and the speed
of light in water is 225,000,000 m/s. What is the refractive
index of water?
1.33
1
15ofof20
39
© Boardworks Ltd 2005
2004
Calculating refractive index
The speed of light in air is 300,000,000 m/s.
The speed of light in crystal is 150,000,000 m/s. What is
the refractive index of crystal?
Refractive index = speed of light in air
speed of light in crystal
Refractive index = 300,000,000
150,000,000
Refractive index of crystal = 2.0
1
16ofof20
39
© Boardworks Ltd 2005
2004
Snell’s law
i
Refractive index = sin i
sin r
Example:
r
air
glass
When a ray passes into a glass
block, i = 45° and r = 28°. What is
the refractive index of the glass?
Refractive index = sin 45
sin 28
Refractive index = 1.5
1
17ofof20
39
© Boardworks Ltd 2005
2004
Contents
Waves: Refraction
Refraction
Refractive index
Refraction effects
Lenses
Summary activities
1
18ofof20
39
© Boardworks Ltd 2005
2004
Effects of refraction
Many visual effects are caused by refraction.
This ruler appears bent
because the light from one
end of the ruler has been
refracted, but light from the
other end has travelled in a
straight line.
Would the ruler appear more
or less bent if the water was
replaced with glass?
1
19ofof20
39
© Boardworks Ltd 2005
2004
Real and apparent depth
The rays of light from a stone get bent (refracted) as they
leave the water.
Your brain assumes
these rays of light
have travelled in
straight lines.
Your brain forms an image
at the place where it thinks
the rays have come from –
the stone appears to be
higher than it really is.
1
20ofof20
39
image
actual location
© Boardworks Ltd 2005
2004
The Archer fish
The Archer fish is a predator that shoots jets of water at
insects near the surface of the water, e.g. on a leaf.
The Archer fish allows
for the refraction of light
at the surface of the
water when aiming at
the prey.
image
of prey
prey
location
The fish does not aim at
the refracted image it sees
but at a location where it
knows the prey to be.
1
21ofof20
39
© Boardworks Ltd 2005
2004
Magic coins
Place a coin in the bottom of a bowl and clamp an
empty cardboard tube so that it points above the coin.
Gradually add water to the bowl and watch the coin through
the tube float up – can you explain this?
1
22ofof20
39
© Boardworks Ltd 2005
2004
Contents
Refraction
Refraction
Refractive index
Refraction effects
Lenses
Summary activities
1
23ofof20
39
© Boardworks Ltd 2005
2004
Refraction and lenses
F
ƒ
1
24ofof20
39
Work
out
how
Draw
normal
Imagine
Draw
normal
The
distance
The
lens
Work
out
how
the
the
ray
isthe
lines
where
parallel
rays
of
lines
(at
90°
to
between
the
refracts
all
rays
are
refracted
refracted
as
it
rays
enter
air
light
from
afor
the
surface)
centre
of
the
rays
to
athe
point
as
the
leave
the
enters
the
lens
(at
90º
to
the
distant
object
each
ray.
lens
and
F
is
called
the
lens
surface).
hitting
the
called
thefocus
focal
principal
lens.
length
(F).().
When light enters a more
less
dense medium (e.g. glass),
air),
it bends towards
away from
thethe
normal.
© Boardworks Ltd 2005
2004
Convex lenses
Convex lenses work by bending (refracting) rays of light
to a principal focus. Convex lenses can be used to
project or magnify images.
The distance from the centre of the lens to the principal
focus (F) is called the focal length (ƒ).
The thicker the lens, the shorter the focal length.
1
25ofof20
39
© Boardworks Ltd 2005
2004
How do light rays pass through lenses
Parallel light rays strike a convex lens
They pass through the
focal point of the lens.
F
Form a parallel beam if they
pass though the focal point (F).
Diverging light rays
F
1
26ofof20
39
© Boardworks Ltd 2005
2004
Finding the focal length of a lens
Hold the lens in the other
hand and move it closer to
the screen until a clear
image appears.
Hold a plain
white screen
in one hand.
Chose
a to
Use a ruler
distant
object
measure
the distance
(to
get parallel
between
the lens and
rays
of light).
the screen
– this is
the focal length (ƒ).
1
27ofof20
39
© Boardworks Ltd 2005
2004
Ray diagram for an object >2F away
Object >2F away
O
2F
F
F
2F
I
The image (l) is formed between F and 2F away from the
lens, and is inverted and diminished.
1
28ofof20
39
© Boardworks Ltd 2005
2004
Ray diagram for an object 2F away
Object at 2F
O
2F
F
F
2F
I
The image (l) is formed at 2F away from the lens, is
inverted and the same size.
1
29ofof20
39
© Boardworks Ltd 2005
2004
Ray diagram for an object between 2F and F
Object between 2F and F away
O
2F
F
F
2F
The image (l) is formed further than 2F away from
the lens, is inverted and magnified.
1
30ofof20
39
I
© Boardworks Ltd 2005
2004
Ray diagram for an object at F
Object at F away
O
2F
F
F
2F
The image (l) is formed at infinity – the rays never meet.
This set up is used for searchlights.
1
31ofof20
39
© Boardworks Ltd 2005
2004
Ray diagram for an object close than F
I
Object between F and lens
O
2F
F
F
2F
The virtual image (l) is
formed on the same side
of the lens as the object.
It is the right way up
and magnified.
1
32ofof20
39
© Boardworks Ltd 2005
2004
Summary of images with a convex lens
Object
position
Image
position
>2F
between F
and 2F
real
diminished
inverted
at 2F
real
same size
inverted
between
2F and F
> 2F
real
magnified
inverted
at F
at infinity
–
–
between F
and lens
same side
as object
at 2F
1
33ofof20
39
Real or
virtual
–
virtual
Magnified Inverted or
or
erect
diminished
magnified
erect
© Boardworks Ltd 2005
2004
Magnification
The magnification factor of a lens can be calculated
by using this equation:
Magnification = height of image
height of object
1
34ofof20
39
© Boardworks Ltd 2005
2004
Contents
Refraction
Refraction
Refractive index
Refraction effects
Lenses
Summary activities
1
35ofof20
39
© Boardworks Ltd 2005
2004
Revision tip
Remember the word:
TAGAGA
Towards (normal)
Air
Glass
Away (from normal)
Glass
Air
1
36ofof20
39
© Boardworks Ltd 2005
2004
Glossary
 convex lens – A lens that brings light rays to a focus.
 focal length – The distance from the centre of the lens to
the principal focus.
 magnification – The size of the image relative to the size
of the original object.
 refraction – The bending of light when it enters a different
material. This happens because light changes direction and
travels at a different speed when it enters different materials.
 refractive index – The ratio of the speed of light in air to
the speed of light in another material.
1
37ofof20
39
© Boardworks Ltd 2005
2004
Anagrams
1
38ofof20
39
© Boardworks Ltd 2005
2004
Multiple-choice quiz
1
39ofof20
39
© Boardworks Ltd 2005
2004