Given:
Separation between the two slits = d
Wavelength of the light =
Distance of the screen =
(a) When the intensity is half the maximum:
Let Imax be the maximum intensity and I be the intensity at the required point at a distance y from the central point.
So,
Here, is the phase difference in the waves coming from the two slits.
So,
(b) When the intensity is one-fourth of the maximum: