Huygen's principal

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   Huygens principal

According to this principal the particles on wave front behaves like fresh source of light emit waves in all direction called secondary wavelets and the envelop (tangential surface) in the forward direction to the secondary wavelets gives the new position of wavefront in the medium. 

   Construction

The light ray passes normally through the wave front and the disturbance propagates with the direction of speed of light in the medium

If disturbance taken 't' time to reach from AB to PQ

It covers, 
                  d = c × t
      Where, d = distance from AB to PQ




Now on drawing sphere of radius ct around particle in PQ
 Then the tangential surface on sphere in forward direction gives a new position A forward wave front. 


Laws of reflection from Huygen's principal






If wavefront taken 't' time from B to C with 'v' velocity
       BC = vt
Taken and arc of radius of vt and draw tangent on it from C. 

Law of refraction




From triangle ABC
   Sini = AB/AC

From triangle ADC
   Sinr = AD/AC

Now,  Sini/Sinr = (BC/AC) / (AD/AC) 

Sini/Sinr = BC/AD

Sini/Sinr = v1× t/ v2 × t

Sini/Sinr = v1/v2 = 





















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