REFRACTION OF LIGHT, Class - 10

 

1.REFRACTION OF LIGHT

The change in direction of light when it passes from one optical medium to another optical medium obliquely,is called refraction of light.
Or,
The bending of light when it goes from one optical medium to another optical medium obliquely, is called refraction of light.
The refraction (bending ) of light takes place at the boundary between the two optical media.`

1.1 Angle of incidence

The angle between incident ray and the normal at the point of incidence is called the angle of incidence.

1.2 Angle of refraction

The angle between refracted ray and the normal at the point of incidence is called the angle of refraction.
N.B.- In the refraction of light, the angle of refraction is usually not equal to the angle of incidence.It is either smaller than or greater than the angle of incidence.

2. CAUSE OF REFRACTION OF LIGHT

The speed of light is different in different media or substances. The change in speed of light on going from one medium to another causes the refraction of light. Greater the difference in the speeds of light in the two media, greater will be the amount of refraction or bending of light.

3.OPTICALLY RARER MEDIUM AND OPTICALLY DENSER MEDIUM

Rarer medium :- A medium in which the speed of light is more or refractive index is less ,is known as optically rarer medium(or less dense medium)
Denser medium:- A medium in which the speed of light is less or refractive index is more , is known as optically denser medium.
a) When a ray of light goes from a rarer medium to a denser medium, it bends towards the normal (at the point of incidence ).

b)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 ).

c) If the incident ray falls normally(or perpendicularly) to the surface of a glass slab, then there is no bending of the ray of light, and it goes straight.

 

4.REFRACTION THROUGH A RECTANGULAR GLASS SLAB

 

A ray of light AO travelling in air is incident on a rectangular glass slab PQRS at point O. On entering the glass slab, it gets refracted along OB and bends towards the normalON'. A second change of direction takes place when the refracted ray of light OB, travelling in glass emerges(comes out) into air at point B. Since the ray of light OB now goes from a denser medium 'glass' into the rarer medium 'air', it bends away from the normal BN'₁ and goes in the direction BC.

• Incident ray AO and emergent ray BC are parallel to each other with some lateral displacement.
• The light emerges from a parallel- sided glass slab in a direction parallel with that in which it enters the glass slab.
• LATERAL DISPLACEMENT:-The perpendicular distance between the original path of incident ray and the emergent ray coming out of the glass slab is called lateral displacement of the emergent ray of light.
• Angle of emergence:-The angle which the emergent ray makes with the normal is called the angle of emergence.If PQ || SR ,then angle of incidence i = angle of emergence e

5.Effects of Refraction of Light

1. A stick or pencil held obliquely and partly immersed in water appears to be bent at the water surface.  
   
   
   
2. An object placed under water appears to be raised. 
   
   
   
3. A pool of water appears to be less deep than it actually is.
4. When a thick glass slab is placed over some printed matter, the letters appear raised when viewed from the top.
   
   
   
5. A lemon kept in water in a glass tumbler appears to be bigger than its actual size, when viewed from the sides.
   
   
   
6. The stars appear to twinkle on a clear night.
   
   
   

6.LAWS OF REFRACTION OF LIGHT

1. 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.
2. 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.( This is true for angle i between 0 and 90⁰ )
   Sin i/Sin r = constant
   This constant value is called the refractive index of the second medium with respect to the first.

7. RERACTIVE INDEX

(A) Absolute refractive index

The ratio of speed of light in vacuum (or in air) to the speed of light in a medium, is called absolute refractive index (or refractive index) of that medium.
Refractive index of a medium = c/v
Where c = 3 × 10⁸ m/s = speed of light in vacuum or air
v = speed of light in a medium