Let the amplitude of light wave coming from the narrower slit be A1 and amplitude of light wave from the wider slit be A2 . As amplitude ∝ Width of slit ∴A2=3A1 Maximum intensity occurs where constructive interference takes place and the minimum intensity where destructive interference takes place. ∴Amax=3A1+A1=4A1 and Amin=3A1−A1=2A1 ∵ Intensity I∝A2 ∴