SKEDSOFT
Classification of Interference Phenomenon: In order to observe interference phenomenon, the two super imposing waves must be originating from the same source, so that the phase difference between them remains constant with time. These waves can be derived by single source using number of methods. Depending on the process by which the waves are derived, these can be classified as follows : i) division of wavefront ii) division of amplitude.
Division of wave front: when the two super imposing waves are derived from the two regions of parent waves, the process is known as division of wave front. Since the phase on the wave front is constant, the waves from two regions of waves will bear a constant phase difference at any point of observations.
An earliest method to obtain two coherent waves (source) by division of wave front of parent wave is young's double slit method and is described below.
Fig.A: Derivation of two coherent waves by method division of wavefront (Young's dubble slit experiment)
In this method, light from broad source is made to fall on a small aperture slot S. Since the aperture is small it acts as a source of cylindrical wave. Now at all points of the wave front, the phase of the wave remains the same. When the light (wave front) falls on Symmetrically placed slits S1 & S2 , these select two different regions of the wave front and light emerging out of these slits have no phase difference. The slits S1 and S2 thus behave as coherent sources. When waves from these sources S1& S2 travel forward and fall on a screen, at any point on screen additional phase difference may arise between two waves depending on the path these waves travel. However resultant phase difference at any point doesn't change with time. The intensity at the point of superposition is decided by the relative phase difference between the two waves.
For a point P(x, y, z) on the screen, the two waves (from S1 and S2) travel distances r1 and r2 respectively. The path difference 
is given by
........................(1)
Where 
d is the separation between slits S1 and S2 .
If the screen is placed at a distance D from the center of double slits (z=0) and D>>d, we have
------------------------------------------(2)
Thus the phase difference between the two waves at the screen will be
-----------------------------------------(3 )
The waves will interfere constructively (bright fringe) or destructively (dark fringe) depending on whether 
is an even or odd multiple of 
. The mth order bright fringe will be at points on the screen where
-------------------------------------------(4)
m=0,1,2,3----------
