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Refraction of Light

Let’s try a very simple activity before we start expansion on the section. Take a transparent glass filled with water, put a pencil inside it and now observe the pencil from outside, how does it look like. It will appear as if pencil has broken into two pieces one lying outside the glass of water and second inside the water but tilted.

Why does it behave like this? It’s refraction.

 Refraction: Whenever a light of ray travels obliquely from one medium (like air) to another medium (like water), its direction of propagation changes. This phenomenon is known as refraction.

 Description of refraction through a glass slab

As shown in the figure above: The incident ray is incident on a glass slab, after entering the glass slab, it undergoes refraction. It deviates from its normal path. It is then incident on the other side of glass slab. Again it encounters a change in a medium which leads to refraction. The two successive refraction encountered by incident ray counterbalances themselves.

Hence the emergent ray is parallel to the incident. It is then incident on the other side of glass slab. Again it encounters a change in a medium which leads to refraction. The two successive refraction encountered by incident ray counterbalances themselves.
Hence the emergent ray is parallel to the incident ray.

Behavior of Refracted Ray:

a) When a ray of light goes from rarer medium to denser medium, it bends towards the normal.
b) When a ray of light moves from denser to a rarer medium, it bends away from normal.
Reason for this behavior:
C=λv
C = speed of light, λ = wave-length of light, v = frequency of light.
When the speed of light enters obliquely in a denser medium, its velocity reduces. The portion of light outside the denser medium continues to travel faster. Hence at the interface of two surfaces, it looks as if ray has bent
Similarly when the ray of light travels, from a denser medium to rarer medium, again at the interface of two surfaces, there is a change in speed of light leading to deviation from normal path.

 

Snell’s law: It says that ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant and this constant is known as the refractive index of the second medium.

Where n is the refractive index? It doesn’t have any units because it’s a constant.
The refractive index can also be expressed as the ratio of the speed of light in both mediums.

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