Class 10th Physics- refraction of light.

Refraction of Light:

You might have observed when any object is immersed in water; it seems to appear bigger. Similarly, when a transparent paperweight is kept on a piece of paper, the letters written on the paper appear to be bigger. How would you explain such a phenomenon? What is the reason for the increase in the size of the object in the first case and letters in the second case?

The entire phenomenon can be understood using the concept of refraction of light. Refraction of light is responsible for this optical effect. Let us see what does refraction of light mean?

When light rays travel from one medium to another, the direction of propagation, as well as the speed of the light rays, alter.The bending of the path of light rays when it travels from one medium to another is known as refraction of light. Though the light bends when it travels from one medium from another, still it travels in a straight path in the given medium. Also, the extent of bending of light rays depends upon the refractive index. The ratio of the speed of light in vacuum to the speed of light in the given medium gives the refractive index of that medium. It is a dimensionless quantity.

n = cv

Where, n = refractive index of the given medium

c is the speed of light in vacuum which is 3 × 108 m/s

v is the speed of light in the given medium

Consider a ray of light AO, incident at the interface of two different media with refractive index n1 and n2 respectively. Let the angle of incidence be θ1 and the angle of refraction be θ2 as shown in fig. 1. Let the speed of light in medium 1 be v1and that in medium 2 be v2.

The refractive index of medium 1 with respect to medium 2 is given as the ratio of speed of light in medium 2 to the speed of light in medium 1.

n12 = n1n2 = v2v1

Similarly,

n21 = n2n1 = v1v2

The bending of light can be related to the refractive indices of two media with the help of Snell’s Law of refraction. According to Snell's law of refraction;

n12 = n1n2 = sin θ1sin θ2

Also, the incident ray, refracted ray and the normal drawn at the point of incidence, all lie in the same plane. If the light is coming from a medium of lighter refractive index to a medium of greater refractive index, the light ray will bend towards the normal. If the light ray is travelling from a medium of higher refractive index to a medium of lower refractive index, then it will bend away from the normal.

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