91Ó°ÊÓ

In a zone-plate, the second, fourth, sixth...zones are blackened; what would happen if instead the \(1^{\text {st }}, 3^{\text {rd }}, 5^{\text {th }}\), etc. zones were blackened?

(a) The output of a He-Ne laser \((\lambda=6328 \AA)\) can be assumed to be Gaussian with plane phase front. For \(w_{0}=1 \mathrm{~mm}\) and \(w_{0}=0.2 \mathrm{~mm}\), calculate the beam diameter at \(z=20 \mathrm{~m}\). (b) Repeat the calculation for \(\lambda=5000 \AA\) and interpret the results physically.