Wave Optics — Part 1: Wavefronts, Huygens’ principle and coherent sources: visual lesson, ISC Class 12 Physics

Lesson

Wave Optics: Interference and Diffraction

Part 1 — light as a wave: wavefronts, Huygens’ principle, and what it takes for two light waves to interfere.

In this lesson you will

Describe spherical, cylindrical and plane wavefronts and use Huygens’ principle to construct a new wavefront
Prove the laws of reflection and refraction with Huygens’ construction, and relate speed, wavelength and frequency in a medium
Relate path difference to phase difference and state the conditions for sustained interference

Concept

Wavefronts and rays

A wavefront is a surface on which every point vibrates in the same phase. Successive wavefronts one wavelength apart move outward at the wave speed.

A ray is a line along which energy travels; in a uniform medium it is always perpendicular to the wavefronts.
The shape of the wavefront depends on the source and the distance from it.

Diagram

Three kinds of wavefront

Table of the three kinds of wavefront: a point source gives spheres with intensity falling as one over r squared, a line source gives cylinders with intensity falling as one over r, a distant source gives planes of constant intensity
Source	Wavefront	Amplitude at distance r	Intensity
Point (small lamp, LED)	Spheres	$A \propto 1/r$	$I \propto 1/r^2$
Line (long tube light, slit)	Coaxial cylinders	$A \propto 1/\sqrt{r}$	$I \propto 1/r$
Very distant (the Sun, a star)	Planes	constant	constant

The same light energy spreads over a larger and larger wavefront, so the intensity falls with distance in a way set by the wavefront’s shape.

Point source: area $4\pi r^2$ , so $I \propto 1/r^2$ .
Line source: area $2\pi r L$ , so $I \propto 1/r$ .
Plane wavefront: area fixed, intensity constant (no spreading).

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