## Refraction through a prism

For an equilateral prism , find two angle of incidence differing by 20 degrees showing same deviation of 40 degrees?

Use the formula $\delta&space;=i_{1}+i_{2}-A$

Here A = 60 degree (Since equilateral prism)

deviation is 40 degree

This gives $A+\delta&space;=i_{1}+i_{2}=100$

Therefore,

the values of angles of incidence are 60 degree and 40 degree

## Refraction – Disappearing letters

A rectangular block of glass is placed on a printed page lying on a horizontal table. what is the minimum value of u of glass so that the letters on page are not visible from any of the vertical faces of the block.

## Light, Time and Vacuum

Why does the light needs time to travel through vacuum?Is there something makes it needs time? How did we prove that  there is nothing in vacuum?

When there is no medium, there is nothing to oppose the propagation of light and therefore it moves with a constant velocity. All electromagnetic waves including light travel in vacuum with the same velocity (3 x 10^8 m/s). Since the energy is to travel from one point to another, a finite time is also required.

The velocity of light is experimentally determined by many different methods. (Romer’s methos, Fizau’s method, Foucault’s method, Michelson’s method etc.)

When light travels in a medium, whenever light interacts with the matter, the atoms absorb light and are re – emitted and the average speed in a medium will be less than in vacuum. In vacuum, there is no matter t interact and that’s why there light travels with maximum velocity.

(If you remember the structure of atom, most of the space inside an atom is vacuum. The speed of light decreases only when it encounters matter. So, when we consider a particular medium, the speed of light is a constant in it)

## Problem from Ray Optics

A diverging beam of light from a point source S having divergence angle alpha falls symetricaally on a glass slab as shown in fig.The angles of incidence of the two extreme rays are equal.If the thickness of the glass slab is t and the refractive index is n,then the divergence angle of the emergent beam is,
a)0
b)alpha
c)sin inverse 1/n
d)2 sin inverse 1/n

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