L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
PHYSICS - GRADE: VIII
REFRACTION OF LIGHT
REFRACTION
When light travels from one transparent medium to another transparent medium, it bends from its
original path. This phenomenon of bending of light is called Refraction.
Refraction takes place at the surface of separation of the two media.
BENDING OF LIGHT TRAVELLING FROM ONE OPTICAL MEDIUM TO ANOTHER
The bending of light ray as it passes obliquely from one optical medium to another takes place
because light travels with different speeds in different media.
DIFFERENCE BETWEEN REFLECTION OF LIGHT AND REFRACTION OF LIGHT
1.
2.
3.
Reflection of Light
Refraction of Light
Reflection of light is defined as the Refraction of light is defined as the
bouncing of light after striking a surface. phenomenon of bending of light when it
travels from one transparent medium to
another transparent medium.
In Reflection of Light, the angle of In Refraction of Light, the angle of incidence
incidence is equal to the angle of is not equal to the angle of refraction.
reflection.
In Reflection of Light, the light ray is In Refraction of Light, the light ray travels
reflected back into the same medium.
from one transparent medium to another
transparent medium.
TERMS RELATED TO REFRACTION OF LIGHT
1. INCIDENT RAY
The ray which falls on the surface of separation to enter into the other medium is known as
the Incident Ray.
2. REFRACTED RAY
The ray obtained in the second medium after refraction is known as the Refracted Ray.
3. NORMAL
A Normal is an imaginary straight line perpendicular to the refracting surface.
4. ANGLE OF INCIDENCE
The angle between the Incident Ray and the Normal at the point of incidence is known as the
Angle of Incidence.
5. ANGLE OF REFRACTION
The angle between the Refracted Ray and the Normal at the point of incidence is known as
Angle or Refraction.
RULES OF REFRACTION
1. When a ray of light passes obliquely from a rarer medium to a denser medium, it always
bends towards the normal. In such case, the angle of refraction is smaller than the angle
of incidence.
2. When a ray of light passes obliquely from a denser medium to a rarer medium, it always
bends away from the normal. In such case, the angle of refraction is greater than the angle
of incidence.
3. When a ray of light passes from one medium to another medium at right angle to the
surface separating the two media, it does not bend. Instead, it goes in its original direction
only.
LAWS OF REFRACTION
1. First Law of Refraction
The incident ray, the normal at the point of incidence and the refracted ray, all lie in the
same plane.
2. Second Law of Refraction
For a given pair of media and given colour of light, the ratio of the since of angle of
incidence to the sine of angle of refraction is a constant.
sin i =
constant
sin r
This constant is denoted by the symbol µ.
The Second law is also known as Snell’s Law.
The constant (µ) is known as the Refractive Index of the second medium with respect to
the first medium.
REFRACTION THROUGH A PARALLEL SIDED GLASS SLAB
Observation
1. The incident ray, the normal and the refracted ray, all are drawn on the same sheet of
paper or, lie on the same plane. Hence, the first law of refraction is verified.
2. The ratio of sin i / sin r is found to be constant for two given media, that is, glass and air.
Hence, Snell’s law is verified (when the experiment is repeated with different angles of
incidence).
3. The angle of incidence and the angle of emergence are always equal. In other words, the
emergent ray is always parallel to the incident ray.
4. The emergent ray shifts from the path of the incident ray. Thus, when a ray of light passes
through a rectangular glass slab, it emerges parallel to the incident ray and is shifted from
its original path.
REFRACTION OF LIGHT THRHOUH A PRISM
Observation:
1. Angle of incidence + Angle of emergence = Angle of prism + Angle of deviation.
2. The incident ray and emergent ray are not parallel to each other. The emergent ray
makes an angle with the extended incident ray (angle of deviation).
3. The angle of deviation depends upon the angle of incidence.
4. When angle of incidence is increased gradually, the angle of deviation first decreases
and attains a minimum value and then increases.
The minimum value it acquires is called as the angle of minimum deviation (dm).
Conditions during minimum deviation:

The angle of incidence and the angle of emergence are equal to each other.

The ray of light, travelling through the prism, is parallel to the base of the prism.
The angle of minimum deviation depends on:

Angle of prism

Material of prism

Angle of incidence

Wavelength and colour of light used.
DISPERSION OF LIGHT
The splitting of white light into its constituent colours as it passes through a refracting medium is
known as Dispersion.
CAUSE OF DISPERSION
The speed of light in a medium is different for the light of different colours. Hence, the refractive
index of the medium is different for the light of different colours. Due to which, they deviate from
their path differently.
SPECTRUM
Spectrum is the band of seven colours obtained on a white screen when white light passes
through a prism and slits into its constituent colours. VIBGYOR constitutes the Spectrum.
RAINBOW
In nature, rainbow is an example of dispersion of white light. After the rain, a large number of
small droplets of water remain suspended in the air. Each drop acts like a small prism. When
sunlight passes through these drops, it splits into seven colours. With a large number of water
droplets contributing to this phenomenon, we get to see a huge continuous band of seven colours
displayed in the sky.
LIGHT DISPERSES THROUGH A PRISM BUT NOT THROUGH A GLASS SLAB
The different coloured light with different frequency bend differently as long as the prism surfaces
on which light is incident and emerging are not parallel. But when the surfaces are parallel, the
effect of dispersion from the incident surface is undone by the other surface. A glass slab acts in
this fashion as it could a join of two prisms which dispersed light from one surface is again undone
by the other surface. Hence, the light disperses through a prism but not through a glass slab.
CONCEPT MAP