Answer
$L=19cm$
Work Step by Step
We are given all of the information needed to find the farthest distance a person can be from the wall, L, using equation (36-17).
$L=\frac{Dd}{1.22 \lambda}$
where D is the diameter of the grains, d is the diameter of the lens, and lambda is the wavelength. Therefore;
$L=\frac{Dd}{1.22 \lambda}$
$L=\frac{2*50*10^{-6}*1.5*10^{-3}}{1.22 *650*10^{-9}}$
$L=0.19m$
$L=19cm$
