Question

A small aperture is illuminated with a parallel beam of λ = 628nm . The emergent beam has an angular divergence of 2∘ . The size of the aperture is.

Answer 1

Relation between an angular divergence(in radians), wavelength and size of the aperture:

$$\alpha = \frac{\lambda}{d}$$

Therefore,

$$d = \frac{\lambda}{\alpha} = \frac{628\text{nm}}{2\cdot \pi / 180} = 18\mu m$$