Question

The radius of the fifth zone of a zone plate is 2 mm. Considering the zone plate as

a converging lens, calculate its focal length for light of wavelength 4800 Å.

Answer 1

Answer

Given data in question

$r_5=0.002m$

n=5

$\lambda=4800\times10^{-10}m$

Thus, the focal length of the zone plate,

$f =\frac{ r_n^2}{n\lambda}\\= \frac{ (0.002)^2}{5\times 4800\times10^{-10} } \\=166.67m$