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. $\therefore \;\; B C=v t$ Let $CE$ represent the tangent plane drawn from the point $C$ to the sphere of radius ' $v t$ ' having $A$ as its center. then $AE=BC=v t$ it follows that $\triangle E A C \cong \triangle B A C$ Hence $\angle i = \angle r$ $\therefore$ 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 $n_{21} = \frac{\; \text{velocity of light in medium}\; 1}{\; \text{velocity of light in medium}\; 2}$ The figure drawn here shows the refracted wave front corresponding to the given incident wave front. It is seen that $\sin i = \frac{BC}{AC} = \frac{v_1 t}{AC}$ $\sin r = \frac{AE}{AC} = \frac{v_2 t}{AC}$ $\therefore \;\; \frac{\sin i}{\sin r} = \frac{v_1}{v_2} = \mu_{21}$ This is Snell's law of refraction.