A small aperture is illuminated with a parallel beam of light of wavelength λ= 628 nm.  The emergent beam has an angular divergence of 20. The size of the aperture is

$$\text{as } \theta \text{ is small} \;\Rightarrow\; a\theta = \lambda$$ $$a = \frac{\lambda}{\theta} = \frac{628 \times 10^{-9}}{3.14} \times 180 = 36 \,\mu m$$