CHAPTER - 5
Einstein was born in 1879 in Germany. He is known throughout the world as a man who propounded the famous theory of relativity it is perhaps ironical that at the time Einstein made his first major contribution to physics, he was working as a clerk in the swiss patent office at Berne.
Einstein was awarded the Nobel Prize in 1921 not for his theory of relativity, but for his services to theoretical physics and especially for his discovery of the law of photo electric effect.
KEY WORDS
| * Refraction | * Incident ray |
| * Refracted ray | * Angle of incidence |
| * Angle of Refraction | * Absolute refractive index |
| * Relative refractive index | * Snell's Law |
| * Critical angle | * Total internal reflection |
| * Mirage | * Shift |
| * Optical fibre | * Interface |
| * Normal | * Optically denser and rarer medium |
KEY WORDS EXPLANATION
Refraction: When light travels from one medium to another, its direction changes at the interface. The phenomenon of changing direction at the interface of the two media is known as refraction.
Incident ray: A ray which falls on the refracting/ reflecting surface from a source of light is known as incident ray.
Refracted ray: A ray which bends towards the normal/ moves away from the normal after entring one medium to another medium is known as refracted ray.
Angle of incidence: The angle between the incident ray and the normal at the point of incidence is known as angle of incidence (i).
Angle of refraction: The angle between the refracted ray and the normal is called angle of refraction (r).
Absolute refractive Index: It is the ratio of speed of light in vacuum (c) to speed of light in the medium (v).
Absolute refractive index n = 
Relative refractive index: Relative refractive index (n21) is the ratio of refractive index of second medium (n2) to refractive index of first medium (n1)
Snell's Law: According to Snell's Law: n1 sin i = n2 sin r
Here n1 is the refractive index of first medium and n2 is the refractive index of second medium. i is the angle of incidence and r is the angle of refraction.
Critical angle: The angle of incidence at which the light ray, travelling from denser to rarer medium, grazes the interface is called the critical angle for densar medium.
Total internal reflection: When the angle of incidence is greater than critical angle, the light ray gets reflected into the denser medium at the interface i.e. light never enters the rarer medium. This phenomenon is called total internal reflection.
Mirage: Mirage is an optical illusion where it appears that water has collected on the road at a distant place, but when we get there, we do not find any water and the road is dry.
Shift: The distance between the parallel rays formed by extending the incident and refracted rays from a glass slab is called shift.
Optical Fibre: An optical fibre is a very thin fibre made of glass (or) plastic having radius about a micrometer (10-6 m). A bunch of such thin fibres form a light pipe.
Interface: The surface separating the two media is called the interface between them.
Normal: The perpendicular line drawn to the surface at the point of incidence/ refraction is called the normal at that point.
Optically denser and rarer medium: In a pair of transparent media, the one that has higher refractive index is called optically denser medium of the two. Similarly the one that has lower refractive index is called optically rarer medium.
SYNOPSIS
* Beauty of the nature is made apparent with light. Light exhibits many interesting phenomenon.
* A coin kept at the bottom of a vessel filled with water appears to be raised.
* The process of changing speed at an interface when light travels from one medium to another resulting in a change in direction is refraction of light.
* Refraction of light involves bending of light ray except when it is incident normally.
* Experiments have showed that refraction occurs due to change in the speed of light in the medium.
* Refractive index is a property of a transparent medium.
* The process of changing speed at an interface when light travels from one medium to another resulting in a change in direction is refraction of light.
* If a ray enters from a rarer medium to dancer medium then refracted ray moves towards the normal drawn at the interface of separation of two media.
* When light travels from denser medium to rarer medium, it bends away from the normal.
* When light ray travels from a rarer medium (air) to a denser medium (glass) the value of r is less than the value of i and the refracted ray bends towards the normal.
* When light ray travels from denser to rarer, it bends away from the normal and r > i.
* Refraction of light occurs according to certain laws.
* n1 sin i = n2 sin r: This is called Snell's Law.
Laws of refraction:
* The incident ray, the refracted ray and the normal to interface of two transparent media at the point of incidence all lie in the same plane.
* During refraction, light follows Snell's Law.
* The angle of incidence at which the light ray, travelling from denser to rarer medium, grazes the interface is called critical angle for denser medium.
* If c is the critical angle then r becomes 90° we get
n12 is called refractive index of denser medium with respect to rarer medium.
* When the angle of the incidence is greater than critical angle, the light ray gets reflected in to the denser medium at the interface i.e. light never enters the rarer medium. This phenomenon is called total internal reflection.
* You can take refractive index of water as 
and the refractive index of glass as 
.
* The formation of a mirage is the best example where refractive index of a medium varies through out the medium.
* Total internal reflection is the main reason for the brilliance of diamonds.
* The critical angle of a diamond is very low (24.4°). So, if a light ray enters a diamond it is very likely to undergo total internal reflection which makes the diamond shine.
* Total internal reflection is the basic principle behind working of optical fibre.
* An optical fibre is a very thin fibre made of glass (or) plastic having radius about a micrometer (10-6 m). A bunch of such thin fibres form a light pipe.
* The important application of fibre optics is to transmit communication signals through light pipes.
* For example about 200 telephone signals, approximately mixed with light waves, may be simultaneously transmitted through a typical optical fibre.
* The clarity of the signals transmitted in this way is much better than the other conventional methods.
* The light ray that falls perpendicular to one side of the slab surface comes out without any deviation.
* The incident and emergent light rays are parallel the distance between the parallel rays is called lateral shift.
Writer: C.V. Sarveswara Sarma
