What is Refraction?
The bending of a light wave when it passes from one medium to another due to the
change in the speed of the light travelling between the two different medium is called
the Refraction of light.
The bending is caused due to the differences in density between the two substances.
Some natural phenomena occurring in nature where refraction of light takes place are
the twinkling of stars, the formation of mirages and Rainbows, optical illusions.
Refraction of light is one of the most commonly observed phenomena, but other
waves like sound waves and water waves also experience refraction. Refraction
makes it possible for us to have optical instruments such as magnifying glasses,
lenses and prisms. It is also because of the refraction of light that we are able to
focus light on our retina.
Diagram of refraction:
I = angle of incidence
r = angle of refraction
e = angle of emergence
What are Rarer and Denser mediums?
 Rarer medium (less dense medium) is a medium in which the density, in terms of
number of particles is comparatively less and hence, speed of light is more. For
example, Air is an optically rarer medium as compared to glass and water.
 Denser medium (Denser medium) is a medium which comparatively has a much
greater density, again in terms of number of particles present, which causes the
speed of light to be less. For example, Glass is an optically denser medium as
compared to air.
Types of Refraction:
The refraction of light occurs in different ways depending on the medium through
which the light travels.

Refraction from rarer to denser medium – When light rays pass through rarer
to a denser medium, the light rays bend towards the normal. Due to this the angle of
refraction is smaller than the angle of incidence. e.g. In the case when light rays pass
from air to water or from air to glass, it bends towards normal. It is because of the
reason that the speed of light rays reduces while passing from air to glass or water.

Refraction from denser to rarer medium – When light rays pass from denser
to rarer medium, the light rays bend away from the normal. Due to this the angle of
refraction becomes more than the angle of incidence. e.g. In case when light rays
pass from water to air or glass to air, light rays bend away from the normal. The
speed of light rays becomes greater while passing from glass or water to air.
Refractive Index:
For a given material or medium, the refractive index is considered the ratio
between the speed of light in a vacuum (c) to the speed of light in the medium (v)
on which it goes. The Refractive index for a medium is represented by small n,
and it is given by the following formula:
n=c/v
where


c is the speed of the light in a vacuum, 3 x108 m/s and
v is the speed of light in any given medium.
Real and Apparent depth:
Real depth is the actual distance from the surface of an object to its deepest point.
Apparent depth is the distance from the surface of an object to the point where it appears
to be located.
Real depth is a measurable quantity and is independent of the observer's position or the
medium through which the object is viewed. For example, the real depth of a swimming
pool is the distance between the surface of the water and the bottom of the pool.
Apparent depth, on the other hand, is affected by the refractive index of the medium
through which the object is viewed and the observer's position. For example, when a stick
is partially submerged in water, it appears to be bent due to the difference in refractive
indices of air and water.
Refractive index, μ = Real Depth
Apparent Depth
Snell’s Law:
According to Snell’s law, the ratio of the sine of the incident angle to the sine of the
refracted angle is a constant, for any light of a given color or for any given pair of
media. The constant value is called the refractive index of the second medium with
respect to the first.
Snell’s Law is given by the relation,
 When a ray of light is travelling from a less dense to denser medium –
n2 = sin i
n1
where,
sin r
i and r are the angle of incidence and refraction,
 n is the refractive index and n 1 and n2 are the refractive indices of medium
1 and 2 respectively, and
 v1 and v2 are the speed of light in medium 1 and 2.

 When a ray of light is travelling from a denser to less dense medium –
n1 = sin r
n2
sin i
Critical Angle:
The critical angle is the angle of incidence for which the angle of refraction is 90°.
The light must travel from an optically denser medium to an optically less dense medium.
is the critical angle.
Refractive index and critical angle:
Sin θc= n2
n1
where, θc is the critical angle
n1 is the first medium through which the ray of light is travelling
n2 is the second medium into which
If the angle of incidence is bigger than this critical angle, the refracted ray will not
emerge from the medium, but will be reflected back into the medium. This is called total
internal reflection.
Total internal reflection is a phenomenon that occurs when a ray of light traveling
through a medium is incident on the boundary of a denser medium at an angle greater
than the critical angle and is thus reflected back into the same medium as the incident ray.
What are the Conditions of Total Internal Reflection?
Following are the two conditions of total internal reflection:


The light ray moves from a denser medium to a less dense medium.
The angle of incidence must be greater than the critical angle
Fibre optics
Total internal reflection is a very useful natural phenomenon since it can be used to
confine light. One of the most common applications of total internal reflection is in fibre
optics. An optical fibre is a thin, transparent fibre, usually made of glass or plastic, for
transmitting light. Optical fibres are usually thinner than a human hair! The construction
of a single optical fibre is shown in the following figure.
The basic functional structure of an optical fibre consists of an outer protective cladding
and an inner core through which light signals travel. The difference in refractive index of
the cladding and the core allows total internal reflection to occur in the same way as
happens at an air-water surface. If light is incident on a cable end with an angle of incidence
greater than the critical angle then the light will remain trapped inside the glass strand. In
this way, light travels very quickly down the length of the cable.
Fibre Optics in Telecommunications
Optical fibres are most common in telecommunications, because information can be
transported over long distances, with minimal loss of data. This gives optical fibres an
advantage over conventional cables.
Fibre optics in medicine
Optic fibres are used in medicine in endoscopes.