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June 18, 2014

Effects of Refraction, Total Internal Reflection & Dispersion - Explained

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Effects of Refraction of Light
  1. A swimming pool always looks shallower than it really is, because the light coming from the bottom of the pool bends when it comes out at the surface due to refraction of light.
  2.  A straight stick which is immersed partly in water always looks to be bent at the surface of water, because the light coming from the stick bends when it comes out at the surface due to refraction of light.
  3.  A coin or stone lying at the bottom of a container filled with water appears to be raised because of refraction of light.
  4.  A line or a spot of ink on a paper always appears to be raised when viewed through the glass slab due to the refraction of light.
  5.  Twinkling of stars is due to the refraction of light.
  6.  Optical illusions such as mirage and looming are also produced due to refraction of light.
 Essential Conditions for Total Internal Reflection
 There are two conditions which are essential for total internal reflection. These are:

1.      The light should travel from a denser medium to a rarer medium.
2.      The angle of incidence of light traveling in denser medium should be greater than the critical angle of the medium

Effects of total internal reflection of Light
Twinkling of Stars: The light rays coming from a star reaches our eyes after passing through the atmosphere having different air layers of different optical densities. But the optical densities of different layers of air keep on changing continuously due to change in temperature conditions. Due to which, the light rays coming from a star are refracted to different amount at different moments of time, and the path of refracted rays keep on changing. As a result, sometimes more light is refracted towards our eyes and the star appears bright to us, whereas sometimes less light is refracted towards our eyes and the star appears dim to us. This gives rise to the twinkling effect of a star.

The Sun is visible to us 2 minutes before actual sunrise and 2 minutes after the actual sunset:  The Sun is visible to us 2 minutes before actual sunrise and 2 minutes after the actual sunset due to atmospheric refraction. Actually when the Sun is slightly below the horizon, then the light rays emitted by the Sun are refracted downwards when passing through the optically rarer air layers into the optically dense air layers of atmosphere. Due to which, the Sun appears to be slightly raised above the horizon and is visible 2 minutes before actual sunrise and 2 minutes after the actual sunset.

Mirage : Mirage is an optical illusion which occurs usually in deserts on hot summer days due to atmospheric refraction and total internal reflection of light rays. In mirage, the object such as a tree appears to be inverted as if it is situated on a bank of a pond of water. 
Looming: Looming is also an optical illusion which occurs usually in very cold regions. In looming, a distant object such as a ship moving in polar areas appears to be hanging in midair due to atmospheric refraction and the total internal reflection of light rays.

Brilliance of diamond: The brilliance of a diamond is due to the total internal reflection of light.   We know that the refractive index of diamond is 2.42, and the critical angle for diamond is 240. The diamond is cut in such a way so that the light which enters the diamond from any face suffers multiple total internal reflections at the various faces before coming out of the diamond. Due to this, the diamond sparkles.

Rainbow (refraction + total internal reflection)

 DISPERSION OF LIGHT

The dispersion of light is the phenomenon of splitting of a beam of white light into its seven constituent colours when passed through a transparent medium. It was discovered by Isaac Newton in 1666. Newton discovered that light is made up of seven different colours. hus the spectrum is a band of seven colours which is obtained by splitting of white light by a glass prism. The order of colours from the lower end of spectrum is violet (V), indigo (I), blue (B), green (G), yellow (Y), orange (O), and red (R). The sequence of the 7 colours so obtained in a spectrum can be remembered by using the acronym ‘VIBGYOR’.

Effects of Dispersion:
a) Prisms and rain drops both create rainbows because of this effect. n the case of a rainbow, the droplets of water on the ground act as the prism, refracting the light twice and dispersing it into all of the colors of the rainbow.
b) Rainbow Formation: The formation of rainbow is based on the process of dispersion of light. It is the most enchanting example of dispersion of light which takes place naturally. Usually a rainbow of seven colours is seen in the sky just after the rain when the Sun is shining. The essential condition to see the rainbow is that the observer must stand with his back towards the sun, when seeing the rainbow.

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