Express angular width of central maximum in terms of wavelength.

Explanation

For a diffraction minima `n lambda=d sin theta`, to find the width of the central maximum, put `n=1` then `sin theta=(lambda)/(d):theta=sin^(-1)((lambda)/(d))`. Here `'theta'` is semi angular width.
For a total angular width `theta'=20=2 sin^(-1)((lambda)/(d))`