A screen is placed at a certain distance from a narrow slit which is illuminated by a parallel beam of monochromatic light. If the wavelength of light used in the experiment is X, and d is the width of the slit, then angular width of central maximum will be

In case of single slit, the intensity falls to zero on both sides of central maximum at an angleθ, given by d sin θ =λ where d = width  and θ is angular separation  between  central maximum and first minimum on either side.So. 2 θ = will be the angular width of central maximum   Hence  θ=sin−1⁡(λ^/d) or 2θ=2sin−1⁡(λ/d)