In a single slit diffraction of light of wavelength λ by a slit of width e, the size of the central maximum on a screen at a distance b is

The direction in which the first minima occurs is θ (say). Then esinθ=λ or eθ=λ or, θ=λe(∵   θ=sinθ  when θ small) Width of the central maximum =2bθ+e=2b.λe+e