Q.What is the meaning of wavefront? Explain the laws of reflection by explaining Huygens' principle of wavefront. OR What is an LED? Explain its principle, construction and working.

A wavefront is a surface of constant phase; Huygens' secondary-wavelet construction reproduces the reflected wavefront and proves $\angle i=\angle r$.

Wavefront: The continuous locus of all particles of a medium that are vibrating in the same phase is called a wavefront. For a point source the wavefronts are spherical; far from the source they become plane. The direction of propagation (the ray) is always perpendicular to the wavefront.

Huygens' principle:

1. Every point on a given wavefront acts as a fresh source of secondary disturbances, sending out secondary wavelets in all forward directions with the speed of the wave.
2. The forward common tangent (envelope) to all these secondary wavelets, after a time $t$, gives the new position of the wavefront.

Laws of reflection by Huygens' principle:

Let a plane wavefront $AB$ be incident on a plane reflecting surface $XY$ at angle $i$. While the wavelet from $B$ travels to $C$ on the surface (distance $BC=v t$), the point $A$ acts as a secondary source and sends a wavelet that, in the same time $t$, reaches $D$ with radius $AD=v t$. The reflected wavefront is the tangent $CD$ from $C$ to the sphere of radius $vt$ about $A$.

 incident      normal      reflected
   \  A________|________ D  /
    \  \       |       /  /
     \  \  i   |   r  /  /
 ------B---\---+---/--C------- reflecting surface XY
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