Describe about diffraction of light?

DefinitionThe bending of the light round the corners of an obstacle and the encroachment in the geometrical shadow is called diffraction.
Or in other words,
When an obstacle comes in the path of waves then the waves get bent at the obstacle the bending of the wave around the corner is called diffraction
Diffraction and Huygen's Principle
According to Huygen’s wave theory every point of wavefront may be regarded as source of light. When the secondary wavelets coming from the various points on the wavefront superimposed together then their interference takes place. Thus due to interference of the secondary wavelets coming from various points of a wavefront the wave phenomena of diffraction occurs.
Classification of Diffraction
Diffraction is classified into two types:
1. Fresnel class Diffraction.
2. Fraunhofer class Diffraction.
1. Fresnel class Diffraction:
In this type of diffraction the source and the screen both are situated at finite distance from the obstacle hence for appoint source the shape of the incident wavefront is spherical and for a slit source the shape of the incident wavefront is cylindrical as the incident wavefront is not reached to all the points of the obstacle at the same instant of time hence the secondary wavelets reaching the various points of optical are not in the same phase for example diffraction of light at straight edge is the example of Fresnel class diffraction
2.Fraunhofer class Diffraction:
In this case the source and the screen both are at infinite distance from the obstacle to obtain this a convex lens is used between the source and the slit such that the source is at the first focus of the lens as a result parallel beams fall on the slit by use of second lens kept between slit and screen such that the screen is at the focus of the lens hence the diffracted beams get focused on the screen hence plane wave fronts reached to the obstacle and the secondary wavelets reaching the obstacle are in same phase for example deffraction of light at a single narrow slit.
Comparative interpretation of Diffraction of sound and light
If we are sitting in side a room, then we can hear the sound of the persons talking outside the room, We can't see them it is so because the sound waves are diffracted by the edges of the doors and the windows and enters into the room but light waves cannot be deffracted by this obstacles.
Actually the wavelength of the ordinary sound is of the order of one metre and the obstacle are also of nearly this order hence the diffraction of the sound by these obstacle is common event but the wavelength of light is very small the normal obstacle are very large as compared to the wavelength of light therefore the diffraction of light is not observed in day to day life the diffraction of light occurs only when the size of obstacle is of order of wavelength of light.