In a single slit diffraction with λ=500nm and a lens of diameter 0.1 mm, width of central maxima, obtain on screen at a distance of 1 m will be

Angle subtended by two minima at the slit in a single slit diffraction= α = 2λ/w
where, w is the slit width.
Here the lens' diameter would act as slit width.
The width of central maxima is the distance between the two minima = dα
where, d is the distance between slit and screen = 1 m
Thus, the width of central maxima = 2 * (500 * 10^-9 m) / (0.1 * 10^-3 m) * 1 m = 10 mm