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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 light.

Causes: 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 × 108> m/s and in glass 2 × 108> m/s. The velocity of light changes while passing from one medium into another medium. So, a ray bends while passing from one medium into another medium. The more the difference in the velocity of light in two different media, the more it bends.
Comparatively, the medium having more density is called a denser medium and the medium having less density is called rarer medium. When a ray of light passes from rarer medium into the denser medium, the angle of incidence is more than the angle of refraction, but when it passes from denser to rarer medium, the angle of incidence is less than the angle of refraction.

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:
  1. The incident ray, refracted ray and the normal lie at a point on the plane of incidence.

  2. 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.

  3. 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) i.e.
    Sin i / Sin r = constant (µ) [µ = a constant factor]

  4. When a ray of light undergoes incidence normally, it doesnot deviate.
Effects of Refraction of Light:

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 internal reflection.
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.

Refractive Index

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 velocities.
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).
Important Table

Material Refractive index with respect to air
Ice 1.31
Water 1.33
Alcohol 1.36
Praffin 1.44
Glass 1.5
Crystal (Ruby) 1.76
Diamond 2.48

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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