Question

Define wave front of a travelling wave. Using Huygens principle, obtain the law of
refraction at a plane interface when light passes from a rarer to a denser medium.​

Answer 1

ANSWER

A wavefront is a surface over which an optical wave has a constant.

B) Let AP be the wavefront of the incident wave, of speed in the first medium be v

1

and in the second medium be v

2

propagating in the direction AA

′

incident on the interface at an angle as shown in fig.

let t be time to travel the wave P to P

′

, in the same time wavefront A will reach at A

′

, 

So,

As per the diagram, Sini=

AP

′

PP

′

∴AP

′

=PP

′

Sin i..................(1)

Similarly AP

′

=AA

′

Sin r.............(2) and, PP

′

=v

1

×t similarly AA

′

=v

2

×t

Now from equation (1) and (2)

v

1

×t Sin i=v

2

×t Sin r

So

Sin 

Sin i

=

v

1

v

2

We know that,

n

1

=

v

1

c

 and n

2

=

v

2

c

So

Sin r

Sin i

=

n

1

n

2

,

This is the snell's rule for refraction.

B) When unpolarized light hits a reflective surface, the vertically polarised aspects are refracted into the surface.The horizontally polarised aspects are reflected off the surface and the light is now perceived as partially polarised.

When light is reflected, there is an angle at which this light is completely polarised. This is called Brewster's angle, after the Scottish physicist who discovered the law.

As per Brewster's ruleμ=tan i

b

,

1.5=tan i

b

∴i

b

=tan

−1

( 23 )

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