Definition of the wavefront
Verification of the law of Reflection
The wave front is defined as a surface of constant phase
Alternatively:The wave front is a locus of points which oscillate in phase.
Consider a plane wave AB incident at an angle ' T on a reflecting surface MN

let t= time taken by the wave front to advance from B to C.
∴‌‌BC=vt
Let CE represent the tangent plane drawn from the point C to the sphere of radius ' vt ' having A as its center.
then AE=BC=vt
it follows that
△EAC≅△BAC
Hence ∠i=∠r
∴ Angle of incidence = angle of reflection
Definition of the refractive index
Verification of laws of refraction
The refractive index of medium 2, w.r.t. medium 1 equals the ratio of the sine of angle of incidence (in medium 1) to the sine of angle of refraction (in medium 2).
Alternatively,
Refractive index of medium 2 w.r.t. medium
n21=‌

‌ velocity of light in medium ‌1

‌ velocity of light in medium ‌2

The figure drawn here shows the refracted wave front corresponding to the given incident wave front.
It is seen that
sin‌i=‌

BC

AC

=‌

v1t

AC

sin‌r=‌

AE

AC

=‌

v2t

AC

∴‌‌‌

sin‌i

sin‌r

=‌

v1

v2

=µ21
This is Snell's law of refraction.