Use Huygen’s geometrical construction to show how a plane wave front at t=0 propogates and produces a wave front later at a time t.

Wavefront : A wavefront is a locus of all particles of medium vibrating in the same phase. 

Huygen’s Principle : There are some phenomena like interference, diffraction and polarisation which could not be explained by Newton’s corpuscular theory. They were explained by wave theory first proposed by Huygen. 

The assumptions of Huygen’s theory are : (i) A source sends waves in all possible directions. The locus of particles of a medium vibrating in the same phase is called a wavefront. For a point source, the wavefront is spherical; while for a line source the wavefront is cylindrical. The distant wavefront is plane. 

Each point of a wavefront acts as a source of secondary wavelets. The envelope of all wavelets at a given instant gives the position of a new wavefront. 

The ratio of sine of angle of incidence and the sine of angle of refraction is a constant and is equal to the ratio of velocities of waves in the two media. This is the second law of refraction, and is called the Snell’s law.

 When a light wave travels from rarer to denser medium, the speed decreases. It does not imply reduction its energy. This is because energy of wave depends on its frequency and not on its speed.