Answer
(a) $f = 8.0~cm$
(b) $D = 1.24~cm$
Work Step by Step
(a) The parallel rays would go through the lens and meet at the focal point.
Therefore, the lens should have a focal length of $8.0~cm$
(b) We can find the minimum lens diameter $D$:
$w = \frac{2.44~\lambda~f}{D}$
$D = \frac{2.44~\lambda~f}{w}$
$D = \frac{(2.44)~(633\times 10^{-9}~m)~(0.080~m)}{10\times 10^{-6}~m}$
$D = 1.24\times 10^{-2}~m$
$D = 1.24~cm$
