REFRACTION OF LIGHT.
Refraction of light refers to the change in direction of
light when it passes from one medium to another.
When a ray of light AO passes from air into a rectangular
block of glass , the light travels in a new direction OI in
the glass and emerges along a direction IB parallel to
AO.
We say that the light is bent or refracted
The direction AO is the direction of the incident ray and
the angel between the incident ray and the normal is
called the angle of incidence i
The direction OI is the refracted ray and the angle
between the refracted ray and the normal is called the
angle of refraction r
The glass is said to be optically denser than the air and r
i
1. When a ray passes from one medium to a more
optically dense medium the ray of light bends or is
refracted towards the normal
2. Conversely when a ray passes from a medium to a
less optically dense medium the ray bends away from
the normal
LAWS OF REFRACTION.
1. The incident ray, the refracted ray and the normal all
lie in the same plane.
2. For a given boundary between two media the ratio of
the sine of the angle of incidence to the sine of the
angle of refraction is a constant
sin i
 constant 1 n 2
sin r
Snells Law
Where 1n 2 is known as the refractive index when the
light travels from medium 1 to medium 2.
It can also be shown that
velocity of light in medium 1
n

1 2
velocity of light in medium 2
If we take medium 1 as air and we know that the
velocity of light in air is 3 x 108 m.s-1 given a special
symbol c where c = 3 x 108 m.s-1
1.
It should be noted that the light is refracted
because light has different velocities in different
media.
2. Usually medium 1 is taken to be air so that if
only one medium is given the equation reads
air nglass 
velocity of light in air
velocity of light in glass
3x10 8
air nglass 
velocity of light in glass
3x10 8
velocity of light in glass 
air nglass
The refractive index is usually written as nglass which
always implies that the first medium is air.
The equations can be used for all mediums
air nwater 
air
nwater
velocity of light in air
velocity of light in water
3x10 8

 nwater
velocity of light in water
3x10 8
velocity of light in water 
nwater
The reversibility of light states that paths of light are
reversible.
Consider a ray of light travelling though air and then
water. We use the angle formed between the light in air
and the normal, which they would call the incident
angle. They also use the angle between the light in water
and the normal, which is called the refracted angle.
What happens if light goes from water into the air? We
imagine a mirror placed on the bottom of the aquarium,
reflecting the laser light back upwards towards the
surface. As we check things out, the pattern followed
resembles the one below: It looks like the pattern upon
entering water is just reversed when light leaves water.
Indeed! If light bends closer to the normal upon entering
water, it bends further from the normal upon leaving it.
This is a nice example of the "reversibility"
air n water  n water 
sin i
1


sin r  sin r 


 sin i 
1
water n air
If the direction in which the ray is travelling is not
mentioned it is assumed that the first medium is AIR.
Some typical values for the index of refraction are given
here:
Substance n
Air
1.00
Water 1.33
Glass 1.50±
Plastic 1.40±
Diamond 2.42
It is known that a pool of water appears more shallow
than it actually is and this is due to the refraction of light,
at the water air boundary.
Consider an object O at an actual depth below the surface
of water with refractive index n. Recall n is the
refractive index from air to water.
A ray travelling through the water makes an angle i with
the normal at the water air boundary.
The ray is refracted at this point along a direction at an
angle r to the normal.
An observer at C viewing the object sees it in the image
position at an apparent depth below the surface of the
water
By reversibility of light
air n water 
sin r
sin i
Which can be shown to be
air nwater 
real depth
apparent depth
PROBLEM SHEET
Take c = 3 x 108 m.s-1
Question 1. A glass plate is 0.6 cm thick and has a
refractive index of 1.55. How long does it take light to
pass through the plate.
Question 2. The speed of light in a certain glass is 1.91 x
108m.s-1. Calculate the refractive index of the glass
Question 3. A ray of light strikes the surface of water at
an angle of incidence of 60o. Calculate the angle of
refraction given
n= 1.33
Question 4. A pool of water is 1 m deep. Calculate its
apparent depth when viewed vertically.
Question 5. A person looking down vertically into a pond
sees a fish apparently 18 cm below the surface.
Calculate the actual depth of the fish in the pond.