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Refraction
 Light waves change speed when they
pass across the boundary between
two substances with different
densities, such as air and glass. This
causes them to change direction and
this effect is called refraction.

Refraction follows a regular pattern. A
ray of light travelling from air into
glass has an angle of incidence that is
usually greater than the angle of
refraction.

There is one special case you need to
know. Refraction doesn't happen if
the waves cross the boundary at an
angle of 90° (called the normal) - in
that case they carry straight on.

Check your understanding of
refraction by having a go at the
animation.
Refractive index
 Refraction happens at the boundary
between two media (substances).
This is because:

when the wave speed decreases, the
wave bends towards the normal

when the wave speed increases, the
wave bends away from the normal

The diagram shows how this works
for light passing into, and then out of,
a Perspex block. The same would
happen for a glass block.
Refractive index
The refractive index of a substance is a
measure of the amount of bending after a
boundary. It is calculated using this
equation:

refractive index = speed of light in a
vacuum/speed of light in the medium

The speed of light in a vacuum is 300
000 000 m/s (3 × 108 m/s).

If the speed of light in glass is 200 000
000 m/s (2 × 108 m/s):

refractive index = 300 000 000/200
000 00 = 1.5

The speed of light in air is close to its
speed in a vacuum, so the refractive
index of air is usually assumed to be 1.
The refractive index of water is 1.33.
(Note that refractive index has no
units.)
Dispersion
When white light passes through a prism,
it is dispersed and the different colours of
the spectrum separate. Dispersion causes
rainbows.
You should be able to recall the spectral
colours in order of wavelength:
Remember: ROYGBIV or ‘Richard Of York
Gave Battle In Vain’.
Dispersion happens because the different
spectral colours travel at the same speed
in a vacuum, but at different speeds in a
medium such as glass. The amount of
bending increases as the change in wave
speed increases. It also increases as the
refractive index increases.
For dispersion in a medium such as glass:
 each spectral colour has a different
refractive index

blue light has a greater refractive
index in a given medium than red
light

blue light refracts (bends) more than
red light
Uses of Total Internal Reflection (TIR)
Some, or all, of the light rays can be
reflected when travelling from glass into
air. Beyond a certain angle, called the
critical angle, all the waves reflect back
into the glass. We say that total internal
reflection or TIR has happened.
Have a go at the animation to check your
understanding of this.
know this law for the examination.
The same thing happens for light rays
travelling from water into air but different
media have different critical angles. For
example, the critical angle for glass is 42°
but for water it is 49°.
Using refractive index data:

Light refracts towards the normal
when it passes from a medium into
another medium that has a higher
refractive index - for example, from
air (1) to glass (1.5).

Light refracts away from the normal
when it passes from a medium into
another medium that has a lower
refractive index - for example, from
glass (1.5) to air (1).
There are many uses of total internal
reflection, including:

optical fibres

binoculars

reflectors on road signs

cat’s eyes on roads
3200000000000000000000000The higher
the refractive index of a medium the
lower its critical angle:
Using speed of light data:

Light refracts towards the normal
when it passes into a medium where
it travels more slowly.

Light refracts away from the normal
when it passes into a medium where
it travels more quickly.
Rearranging the equation for refractive
index
You should be able to rearrange this
equation:
refractive index = speed of light in a
vacuum/speed of light in the medium
Total internal reflection happens when
the angle of incidence is greater than the
critical angle.
Predicting the direction of refraction
You should be able to use refractive index
data and the speed of light to predict the
direction of refraction. This is described
by Snell’s Law - but you do not need to
For example, what is the speed of light in
glass, assuming that the refractive index
of glass is 1.5 and the speed of light in a
vacuum is 3 × 108 m/s?
= 2 x 108 m/s