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 $\mathit{A}\mathit{B}$ incident at an angle ' $\mathit{T}$ on a reflecting surface $\mathit{M}\mathit{N}$

let $\mathit{t}=$ time taken by the wave front to advance from B to C.
$\therefore \mathit{B}\mathit{C}=\mathit{v}\mathit{t}$
Let $\mathrm{CE}$ represent the tangent plane drawn from the point $\mathrm{C}$ to the sphere of radius ' $\mathit{v}\mathit{t}$ ' having $\mathrm{A}$ as its center.
then $\mathrm{AE}=\mathrm{BC}=\mathit{v}\mathit{t}$
it follows that
$△\mathit{E}\mathit{A}\mathit{C}\cong △\mathit{B}\mathit{A}\mathit{C}$
Hence $\angle \mathit{i}=\angle \mathit{r}$
$\therefore$ 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


The figure drawn here shows the refracted wave front corresponding to the given incident wave front.
It is seen that
$\mathit{s}\mathit{i}\mathit{n}\mathit{i}=\frac{\mathrm{BC}}{\mathrm{AC}}=\frac{{\mathit{v}}_1\mathit{t}}{\mathrm{AC}}$
$\mathit{s}\mathit{i}\mathit{n}\mathit{r}=\frac{\mathrm{AE}}{\mathrm{AC}}=\frac{{\mathit{v}}_2\mathit{t}}{\mathrm{AC}}$
$\therefore \frac{\mathit{s}\mathit{i}\mathit{n}\mathit{i}}{\mathit{s}\mathit{i}\mathit{n}\mathit{r}}=\frac{{\mathit{v}}_1}{{\mathit{v}}_2}={\mathit{\mu }}_{21}$
This is Snell's law of refraction.