Interference:

When teo light easves travels in a same region, then there will be change in intensity due to superposition of light waves. This phenomenon is called interference. In other world non uniform distribution of energy when two light source super impose.

COndition for maxima and minima
Derive an expression for intensity on different points
let us consider two light waves travelling in a same region
let, y1 = asinwt ------1
y2 = ain(wt + theta)--------2
be the equation of two waves. We considered same amplitude, angular velocity but slightly different phase.
Here phase difference = theta
Now, the resultant wave can be given as
y = y1 + y2
= asinwt + asin(wt + theta)
=a(sinwt + sin(wt + theta))
y=(2acos(theta/2)*(sinwt+(theta/2))-------3
here, A = 2acos(theta/2)
We know intensity is directly proportional to A^2
therefor I directly porportional to A^2
I = A^2
I = 4a^2(cos(theta/2))

for maxima,
cos^2(theta/2) = 1
cos(theta/2)=+-1
cos(theta/2) = cos(n(pie)), n = 0,1,2....
or theta/2 = n(pie)
therefore theta = 2npie
here theta phase difference = lamda/2pie * theta phase difference = n*lamda

for minima cos^2(theta/2) = 0
or cos(theta/2) = 0
cos(theta/2) = cos(2n+1/2)pie
theta = (2n+1)pie

corresponding path difference
= lamda/2pie *(2n + 1)/2*pie
=(2n+1)/2 * lamda
maximum point 0, lamda,2lamda,3lamda
minimum points lamda/2 ,3/2lamda 5/2lamda