(a) If the width of each slit is comparable to the wavelength of light used, the interference pattern thus obtained in the double-slit experiment is modified by diffraction from each of the two slit.
(b) Give that Wavelength of the light beam, λ1=590nm=5.9×10−7m
Wavelength of the light beam, λ2=596nm=5.96×10−7m
Distance of the slit from the screen =D+1.5m
Distance between the two slits, a=2×10−4m
For the first secondary maxima, sinθ=3λ12a=xiD
X1=3λ1D2aandX2=3λ2D2a
X1=3×590×10−9×152×2×10−4,X2=3×596×10−9×1.52×2×10−4
X1=663.75×10−5,X2=670.50×10−5
∴ Spacing between the position of first secondary maxima of two sodium lines
X2−X1=6.75×10−5m=0.0675mm