Answer
$L=160m$
Work Step by Step
We know that:
$\theta=\frac{y}{L}=1.22(\frac{\lambda}{d})$
This can be rearranged as:
$L=\frac {yd}{(1.22\lambda)}$
We plug in the known values to obtain:
$L=\frac{0.02\times 0.005}{(1.22\times 500\times 10^{-9})}=160m$
