CBSE Class 12-science Answered

In the above fig.,

AB is incident plane wavefront

A'B' is refracted wavefront

PQ is refracting surface

B'A'A'' represents the tangential plane to the secondary wavelet

Let a plane AB be incident on a refracting plane surface PQ separating a rarer medium of refractive index  from a denser medium of refractive index .

During the time the disturbance from B reaches B', the disturbance from A must have travelled a distance AA' = c₂t, where 'c₂' is the velocity of light in denser medium and 't' is the time taken by the disturbance to travel from B to B'. Also, BB' = c₁t, where c₁ is the velocity of light in the rarer medium.

With A as centre and AA' as radius, draw a sphere. Draw a tangent to the sphere, from point B'. B'A' will be the refracted wavefront.

Let us now confirm the validity of the refracted wavefront. For A'EB' to be the true refracted wavefront, the following should be satisfied.