Please explain the hugyen's principle?

Huygen's Principle

The first proposer of the wave theory of light, Huygens, considered light to be a mechanical wave moving ina hypothetical medium whcih was named as ether. If we consider a surface a enclosing a light source S, the optical disturbance at any point beyond a must reach after crossing a. The particles of the surface a  vibrate as the wave from S reaches there and these vibrations cause layer beyond to vibrate> we can thus assume that the particles on a act as new sources of light waves emitting spherical waves and disturbance at a point Z beyond a , is caused by the superposition of all these spherical waves coming from different points of a. Huygens called the particles spreading the vibrations beyond them as secondary sources and the spherical wavefronts emitted from these secondary sources as secondary wavelets.

Huygens principle may be stated in its general form as follows :

Various points of an arbitrary surface, when reached by a wavefront, become secondary sources of light emitting secondary wavelets. The disturbace beyond the surface results from the superposition of these secondary wavelets.

the Hugyne's principle is:

Every point on a light wave acts as a secondary source and produces a secondary wave front,

an envelope of these secondary wave fronts gives us the shape of primary wave front

eg:     

The Huygens–Fresnel principle is a method of analysis applied to problems of wave propagation . It recognizes that each point of an advancing wave front is in fact the center of a fresh disturbance and the source of a new propagation of waves; and that the advancing wave as a whole may be regarded as the sum of all the secondary waves arising from points in the medium already traversed.

For example, if two rooms are connected by an open doorway and a sound is produced in a corner of one of them, a person in the other room will hear the sound as if it originated at the doorway. As far as the second room is concerned, the vibrating air in the doorway is the source of the sound. The same is true of light passing the edge of an obstacle, but this is not as easily observed because of the short wavelength of visible light