Hi,

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 :

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.  Proof of Snell’s law of Refraction using Huygen’s wave theory : When a wave starting from one homogeneous medium enters the another homogeneous medium, it is deviated from its path. This phenomenon is called refraction. In transversing from first medium to another medium, the frequency of wave remains unchanged but its speed and the wavelength both are changed. Let XY be a surface separating the two media ‘1’ and ‘2’. Let v1 and v2 be the speeds of waves in these media.

Hope this helps.