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Refraction through a Rectangular Glass Slab

Take a rectangular glass slab PQRS as shown in the figure. The glass slab is surrounded by air. Hence, we have two different media; one is air and the other is glass. Glass is an optically dense medium when compared to air. Now, a ray of light AO traveling in air is incident on the glass slab at point O.

Since the glass slab is transparent, most of the incident light passes through the glass slab. Glass is also an optically denser medium when compared to air. So when the ray of light AO passes from air into glass, its direction changes. It goes along the line OB inside the glass slab and we say that the light ray has been refracted, because AO and OB are not in the same straight line. Refraction of light takes place at the boundary between the two media. In this case, the refraction of light takes place at point O, the boundary of the two media, air and glass.

Angle of Incidence and Angle of Refraction
Let us now define the angle of incidence and the angle of refraction. For the light passing from air into glass, AO is the incident ray and OB is the refracted ray. Let us draw a normal NON’ at the point of incidence O. Now, the angle between the incident ray and the normal is called the angle of incidence. In this case, AON is the angle of incidence. The angle of incidence is denoted by the letter i.

On the other hand, the angle between the refracted ray and the normal is called the angle of refraction. In the figure, N’OB is the angle of refraction. The angle of refraction is denoted by the letter r.

Now you are familiar with the refraction of light. Refraction is due to change in the speed of light as it enters from one transparent medium to another. Experiments show that refraction of light occurs according to certain laws.

Laws of Refraction
The incident ray, the refracted ray and the normal to the interface of the two transparent media at the point of incidence, all lie in the same plane.

The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media.

If ‘i’ is the angle of incidence and ‘r’ is the angle of refraction, then


This law is also known as Snell’s Law of Refraction.


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