Snell’s law of refraction using Huygen’s Principle :
Consider a plane wavefront AB incident on a surface PQ separating two media 1 and 2.

The medium 1 is a rarer medium of refractive index n1 in which light travels with a velocity C1 The medium 2 is a denser medium of refractive index n2 in which light travels with a velocity C2.

The angle between the incident ray FA and the normal NA at the point of incidence A is equal to i.

The angle is also equal to the angle between the incident plane wavefront BA and the surface of separation PQ.

So ∠BAD is the angle of incidence of the incident plane wavefront AB.

Similarly, the angle between the refracted wavefront and the surface of separation PQ is equal to the angle of refraction r.

i.e.. ∠ADC = r.

Consider the triangles BAD and ACD figure above.