CBSE Class 12 · 2003 · Wave OpticsShort Answer

$\text{Wavefront:}$ the locus of all points of a medium oscillating in the same phase.
$\text{Shape from convex lens:}$ a point source at the focus of a convex lens produces a parallel emergent beam, so the emergent wavefront is $\text{plane}$.
$\text{Law of reflection (Huygens):}$ let a plane wavefront $AC$ strike a reflecting surface $XY$, with $A$ reaching the surface first. While the secondary wavelet from $A$ travels a distance $AD$ in the medium, the disturbance at $C$ travels $CE=AD$ to the surface. Draw the reflected wavefront $DE$ (tangent to the wavelet of radius $AD$ centred at $A$).

Hence the triangles are congruent, giving $\angle CAD=\angle EDA$, i.e.

so the angle of incidence equals the angle of reflection.