DIFFRACTION AND INTERFERENCE OF LIGHT

Light is a transverse electromagnetic wave.  Reflection, refraction, diffraction, and interference are phenomena observed with all waves.   

A periodic mechanical wave is a periodic disturbance that moves through a medium.  The medium itself goes nowhere.  The individual atoms and molecules in the medium oscillate about their equilibrium position, but their average position does not change.  As they interact with their neighbors, they transfer some of their energy to them.  The neighboring atoms in turn transfer this energy to their neighbors down the line.  In this way the energy is transported throughout the medium, without the transport of any matter.

Each point on a wavefront can therefore be considered a point source for the production of new waves.  In three dimensions, these new waves are spherical waves called wavelets, that propagate outward with the speed characteristic of waves in the medium.  The wavelets emitted by all points on the wavefront interfere with each other to produce the traveling wave.  This is called Huygen's principle.  It also holds for electromagnetic waves.  When studying the propagation of light, we can replace any wavefront by a collection of sources distributed uniformly over the wave front, radiating in phase.

When light passes through a small opening, comparable in size to the wavelength l of the light, in an otherwise opaque obstacle, the wavefront on the other side of the opening resembles the wavefront shown below.

The light spreads around the edges of the obstacle.  This is the phenomenon of diffraction.