CBSE Class 12 · 2019 · Wave OpticsShort Answer

$\text{Wavefront:}$ the surface (locus) joining all points of a medium that are vibrating in the same phase at a given instant.
$\text{Law of reflection:}$ let a plane wavefront $ABC$ be incident at angle $i$ on a reflecting surface $AD$, the wave speed being $v$. In time $\Delta t$ the disturbance from $C$ reaches $D$ ($CD=v\Delta t$), while the secondary wavelet from $A$ spreads a radius $v\Delta t$; the common tangent $DEF$ from $D$ is the reflected wavefront.

By the RHS rule the triangles are congruent, so $\angle CAD=\angle FDA$, i.e.

Hence the angle of incidence equals the angle of reflection, and the incident ray, reflected ray and normal are coplanar.