**Refraction of a plane wavefront at a plane surface**

Let XY be a plane refracting surface separating two media 1 and 2 of refractive indices μ_{1} and μ_{2} (Figure). The velocities of light in these two media are respectively c_{1} and c_{2}. Consider a plane wave front AB incident on the refracting surface at A. PA and QBC are perpendiculars drawn to AB at A and B respectively. Hence they represent incident rays. NAN_{1} is the normal drawn to the surface. The wave front and the surface are perpendicular to the plane of the paper.

**Fig: Refraction of a plane wavefront at the plane surface.**

According to Huygen’s principle each point on the wave front act as the source of secondary wavelet. By the time, the secondary wavelets from B, reaches C, the secondary wavelets from the point A would travel a distance AD = C2t, where t is the time taken by the wavelets to travel the distance BC.

**∴ BC = C _{1}t and AD = C_{2}t = C_{2 }BC/C_{1}.**

Taking A as centre and C_{2 }BC/C_{1 }as radius an arc is drawn in the second medium. From C a tangent CD is drawn to this arc. This tangent CD not only envelopes the wavelets from C and A but also the wavelets from all the points between C and A. Therefore CD is the refracted plane wavefront and AD is the refracted ray.