Consider a plane surface KY separating a rarer medium of refractive index n1 from a denser medium of refractive index n2 Let c1 and c2 be the values of velocity of light in the two media. AB is a plane wave front incident on XY at an angle i. Let at a given instant the end A of wave front just strikes the surface XY but the other end B has still to cover a path BC. If it takes time t, then BC = c1t.
According to Huygens’s principle point A meanwhile begins to act as secondary source of light and secondary wavelets from it will cover a distance c2t in second medium in time t. Draw a circular arc with A as center and c2t as radius and draw a tangent CD from point C on this arc. Then CD is the refracted wave front, which advances in the direction of rays 1,2. The refracted wave front subtends an angle r from surface XY.
which is Snell’s law of refraction.
