Explain the reflection of a plane wave using Huygen's principle. 

Consider a plane wave $\mathrm{AB}$ incident at an angle $i$ on a reflecting surface $\mathrm{MN}$.

The velocity of wave in medium is $v$ and $\tau$ is time to move the wavefront from point $\mathrm{B}$ to $\mathrm{C}$. $\therefore \mathrm{BC}=v \tau$

As shown in figure plane wave $\mathrm{AB}$ is incident on reflective surface $\mathrm{MN}$ and its reflective wavefront

is $\mathrm{CE}$.

In figure $\triangle \mathrm{EAC}$ and $\triangle \mathrm{BAC}$ are similar triangles,

Here, $\mathrm{AE}=\mathrm{BC}=v \tau$

$\angle \mathrm{AEC}=\angle \mathrm{ABC}$

and $\mathrm{AC}=\mathrm{AC}$

hence $\angle \mathrm{BAC}=\angle \mathrm{ECA}$

$\therefore i=r$ is the law of reflection.