## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

(a) $f = 8.0~cm$ (b) $D = 1.24~cm$
(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 $D$: $w = \frac{2.44~\lambda~f}{D}$ $D = \frac{2.44~\lambda~f}{w}$ $D = \frac{(2.44)~(633\times 10^{-9}~m)~(0.080~m)}{10\times 10^{-6}~m}$ $D = 1.24\times 10^{-2}~m$ $D = 1.24~cm$