Distribution Of Intensity On Screen
Distribution of intensity on screen: The equation for intensity on screen for double slit contains two terms. One of these term is 
,which represents the distribution due to pure interference effects and the other 
is due to diffraction.
The intensity at screen will be zero whenever either of 
or 
is zero i.e. whenever
or 
By definition
fig..(1)
fig..(2)
hence first series of minima occurs, when 
(interference minima) or , 
The second series of minima occur when
(diffraction minima)
or, 
As in the case of single slit diffraction, here also exact position of maxima is not defined(except the principal maxima). Approximate position of these maxima can be determined by neglecting 
term and getting the condition of maxima by making 
ie. for 
or 
(interference maxima)
Here p represent the order of interference. The resultant intensity distribution in the case of diffraction by double slit is given by that in fig.3 . We can see that resultant intensity distribution in the case comprises on several peaks within the envelope formed by the diffraction term 
. There are several interesting features in this intensity distribution pattern. The intensity goes to zero when either of the term is zero or both are zero. The second condition is satisfied when