Q16 of 26 Page 1

(i) Derive Snell’s law on the basis of Huygen’s wave theory when light is travelling from a denser to a rarer medium.

(ii) Draw the sketches to differentiate between plane wave-front and spherical wave front.


Let v1 and v2 are the speeds of light in both mediums, AB be the plane wave-front propagating in the direction of A’A incident at an angle I and τ be the time taken by the wave-front to travel the distance BC. So, .


To determine the shape of the shape of the wave-front, draw a sphere of radius v2τ from A in medium2. Let CE be tangent plane drawn from C on to the sphere representing refracted wave-front. So, . Consider the triangles ABC and AEC,




⇒


∴ If r>i (ray bends away from normal), v2>v1.


Now if c be the speed of light in vacuum,



And


So,


∴( this is Snell’s law of refraction)


(ii) Sketches to differentiate between plane wave-front and spherical wave-front:



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