× m s – to all the points on the wavefront. Thus, the refracted wavefront ′ ′ A B emanates as a plane wavefront. The two normals N and ′ N are considered at the points where the rays L and M fall on the refracting surface. As refraction happens from rarer medium ( ) to denser medium ( ), the speed of light is v and v before and after refraction and v is greater than v , ( v > v ).
But, the time taken t for the ray to travel from B to ′ B is the same as the time taken for the ray to travel from A to ′ A . t BB AA BB AA ′ = ′ ′ ′ = (or) (i) The incident rays, the refracted rays and the normal are in the same plane. (ii) Angle of incidence, i = ∠ NAL = o – ∠ NAB = ∠ BAB ′ Angle of refraction, r = ∠ ′ ′ ′ N B M = o – ∠ ′ ′ ′ N B A = ∠ ′ ′ A B A For the two right angle triangles ∆ ABB ′ and ∆ AA'B' , sin sin / / i r BB AB AA AB BB AA ′ ′ ′ ′ = ′ ′ = / / c c v v = v v c c Here, c is speed of light in vacuum. The ratio c / v is a constant, called refractive index of the medium.
The refractive index M' M L' L B' N' N A' i i r r r X Y Medium( ) Medium( ) Figure . Law of refraction - - - - Unit wave optics INTERFERENCE The phenomenon of superposition of two light waves which produces increase in intensity at some points and decrease in intensity at some other points is called interference of light. Superposition of waves refers to addition of waves. The concept of superposition of mechanical