Chapter 8 - Light
Light is the form of energy that gives us the sensation of
vision to our eyes.
And the study of light is known as optics.
Do you notice
a pencil after dipped partially
into a glass containing water, what difference in shape have you
found there? Lets study
deeply on those topics.
The phenomenon of bending of light when it goes from one
optical medium to another is called refraction of
The velocity of light is different in different media. The
velocity of light in
air or vacuum is 3 × 108 m/s, in water 2.2 ×
m/s and in glass 2 × 108>
m/s. The velocity of light changes while passing from one medium
another medium. So, a ray bends while passing from one medium
another medium. The more the difference in the velocity of light
different media, the more it bends.
Comparatively, the medium having more density is called a denser
and the medium having less density is called rarer medium. When
of light passes from rarer medium into the denser medium, the
incidence is more than the angle of refraction, but when it
denser to rarer medium, the angle of incidence is less than the
Laws of Refraction of Light:
When a ray of light undergoes refraction, it abides by certain
rules and are called as laws of refraction of light. These are:
Effects of Refraction of Light:
The incident ray, refracted ray and the normal lie at a
point on the plane of incidence.
When a ray of light goes from rarer to denser medium it
bends towards the normal and when it goes from denser to
rarer medium it bends away from the normal.
The ratio of sine of angle of incident (sin i) to the
sine of angle of refraction (sin r) for a particular
pair of medium always remains same (snell's law)
Sin i / Sin r = constant (µ) [µ = a
When a ray of light undergoes incidence normally, it
i. A pond seems shallower than its actual depth.
ii. A pencil partially dipped into water appear to be bent.
iii. The stars seems twinkling at night.
Total Internal Refraction of Light:
When a ray of light passes from a denser medium to a
rarer medium with the angle of incidence more than
the critical angle, the ray is reflected back into the same
medium instead of refraction. This process is called total
The angle of incidence in a denser
medium for which the corresponding angle of refraction in a
rarer medium is 90°, is called the critical angle.
It is the ratio of velocity of light in vacuum to the velocity of
light in the given medium.
Refractive index (µ)= velocity of light in vacuum / velocity of
light in medium.
It doesn't have any unit as it is simply a ratio of two
The refractive index of a pair of mediums can also be expressed by
the ratio of sine angle of incidence (sin i) to the sine angle of
refraction (sin r).
|Refractive index with respect to air
Dispersion of Light
The process of splitting a chromatic light into corresponding
visible colours due to unequal deviation when it passes through a
prism is called dispersion of light.
The visible ray of light is a chromatic light
that possesses different visible colours with different wave
lengths. When a ray of visible light refracts through a
prism, it bends towards the base inside the prism. As it
emerges out from prism, it again bends towards the base.
Because of different wave lengths of visible colours, they
are split of and dispersion of light occurs.
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