Refraction of Light
- We know that the light travels in a straight path
- However, this holds true upto it travels through a same media.
- When light rays change the medium, the direction also changes
- The change in direction of light when it passes from one medium to another obliquely, is called refraction of light.
What causes Refraction
- The refraction of light is due to the change in the speed of light on going from one medium to another.
- Greater the difference in the speeds of light in the two media, greater will be the amount of refraction (or bending) of light.
- That the speed of light waves on one side of a beam of light changes a little before the change in speed of light waves on its other side, causes a change in the direction of light.
- A medium in which the speed of light is more is known as optically rarer medium (or less dense medium).
- Air is an optically rarer medium as compared to glass and water.
- A medium in which the speed of light is less, is known as optically denser medium
- When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (at the point of incidence)
- When a ray of light goes from a denser medium to a rarer medium, it bends away from the normal (at the point of incidence).
Laws of Refraction
The following are the laws of refraction of light.
(i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane.
(ii) 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. This law is also known as Snell’s law of refraction.
If i is the angle of incidence and r is the angle of refraction, then
Snell’s law is not applicable when angle of incidence is equal to zero, i.e., when the ray is incident along the normal
Refractive index of a medium
- Refractive index of a medium is the ratio of velocity of light in air or vacuum to its velocity in a given medium.
- This is known as the absolute refractive index
- The speed of light is higher in a rarer medium than a denser medium.
- Thus, a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal.
- When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal
EFFECTS OF REFRACTION OF LIGHT
(i) a stick (or pencil) held obliquely and partly immersed in water appears to be bent at the water surface.
(ii) an object placed under water appears to be raised.
(iii) a pool of water appears to be less deep than it actually is.
(iv) when a thick glass slab is placed over some printed matter, the letters appear raised when viewed from the top.
(v) a lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
(vi) the stars appear to twinkle on a clear night.