Refraction of Light

Refraction of Light

When a ray of light propagating in a medium enters the other medium, it deviates from its path. This phenomenon of change in the direction of propagation of light at the boundary when it passes from one medium to other medium is called refraction of light.
When a ray of light enters from rarer medium to denser medium (as from water to glass) it deviates towards the normal drawn on the boundary of two media at the incident point. Similarly in passing from denser to rarer medium, a ray deviates away from the normal. If light is incident normally on the boundary i.e. parallel to normal, it enters the second medium undeviated.

Laws of refraction

1. Incident ray, refracted ray and normal drawn at incident point always lie in the same plane.
2. Snell's law : For a given colour of light, the ratio of sine of angle of incidence to the sine of angle of refraction is a constant.
»   Absolute refractive index of a medium is defined as the ratio of speed of light m free space (vacuum) to that in the given medium.
i.e. absolute refractive index = Speed of light in vacuum / Speed of light in the medium
»   The refractive index of a medium is different for different colours. The refractive index. If a medium decreases with the increase in wavelength of light Hence refractive index of a medium is maximum for violet colour of light and minimum for red colour of light.
»   The reflective index of a medium decreases with the increase in temperature, But this variation is very small.
»   When a ray of light enters from one medium to other medium, its frequency and phase do not change but wavelength and velocity change.

Some illustrations of Refraction
1.   Bending of a linear object when it is partially dipped in a liquid inclined to the surface of the liquid.
2.   Twinkling of stars
3.   Oval shape of sun in the morning and evening.
4.   An object in a denser medium when seen from a rarer medium appears to be at a smaller distance.

This is way (a) A fish in a pond when viewed from air appears to be at a smaller depth them actual depth (b) A coin at the base of a vessel filled with water appears raised.

Critical angle

In case of propagation of light from denser to rarer medium through a plane boundary, critical angle is the angle of incidence for which angle of refraction is 90°.

Total Internal Reflection

If light is propagating from denser medium towards the rarer medium and angle of incidence is more than critical angle, then the light incident on the boundary is reflected back in the denser medium, obeying the laws of reflection. This phenomenon is called total internal reflection as total light energy is reflected, no part is absorbed or transmitted.

For total internal reflection
1. Light must be propagating from denser to rarer medium.
2. Angle of incidence must exceeds the critical angle.

Illustrations of total internal reflection
1. Sparkling of diamond
2. Mirage and looming.
3. Shining of air bubble in water.
4. Increase in duration of sun's visibility-The sun becomes visible even before sun rise and remains visible even after sunset due to total internal reflection of light.
5. Shining of a smoked ball or a metal ball on which lamp soot is deposited when dipped in water.
6. Optical fibre consists of thousands of strands of a very fine quality glass or quartz (of refractive index 1.7), each strand coated with a layer of material of lower refractive index (1.5). In it, light is propagated along the axis of fibre through multiple total internal reflection, even though the fibre is curved, without loss of energy.
1. For transmitting optical signals and the two dimensional pictures.
2. For transmitting electrical signals by first converting them to light.
3. For visualising the internal sites of the body by doctors in endoscopy.