Refraction

1
Refraction
The speed of light in a vacuum is 3x108 ms-1
When light enters a more dense material, (e.g.
perspex), it will slow down
and because of this
it will change direction
towards the normal

• This is called Refraction.

2
As the light returns to the less dense air, it
will change direction away from the normal

• The change in speed (and hence direction) depends
  upon the index of refraction, n of the materials
  concerned.

3
Typical Refractive Indices
4
Snell's Law
For a light ray moving from material 1 into
material 2, the change in direction is given by
n1
n2
Material 1
Material 2
where n1 is the refractive index of material 1,
and is the angle between the incident ray
and the normal in material 1.
5
Example 1
A light ray enters a perspex block at 30o to the
normal. Calculate the angle of refraction in the
perspex.
na 1.00 np 1.50
6
Determining Refractive Index by Experiment
Perspex D Block
D-
7
Providing that the light ray is moving from air
to another material (e.g. perspex), the following
will be true
and therefore
y m x
A straight-line graph can be obtained, which
should go through the origin, with a gradient
equal to the refractive index of the perspex.

• This special case is only true if air is the
  first material!

8
Effect on Frequency and Speed
The frequency of a wave is entirely determined by
the source of that wave. The frequency of a wave
does not change after leaving its source.
The speed of a light ray will change on entering
a different material. The change in speed is
described by a variation of Snells Law
n1v1 n2v2
9
Effect on Wavelength
Because the speed changes, the wavelength of the
light ray must also change.
This is because the frequency, wavelength and
speed are linked by the Wave Equation

• Speed Frequency x Wavelength

If speed decreases, and frequency is constant,
the wavelength must also decrease.
n1l1 n2l2
10
Example 2
A red light ray (633 nm) enters a glass block at
30o in air. The angle of refraction in the glass
is measured as 17o. Calculate
a) The refractive index of the glass
block. b) The speed and wavelength of the light
ray in the glass block.
a)
11
b)