Question

A zone plate of focal length $60\, cm$ behaves like convex lens, if wavelength of incidence light is $6000\, \mathring{A}$, then radius of first half period zone will be :

• A. $ 36\times 10^{-8}\,m$
• B. $ 6\times 10^{-8}\,m$
• C. $ \sqrt{6}\times 10^{-8}\,m$
• D. $ 6\times 10^{-4}\,m$

Answer 1

Answer: (D)

Radius of half period zone $=\sqrt{n b \lambda}$
Radius of Ist zone
$=\sqrt{1 \times 60 \times 10^{-2} \times 6 \times 10^{-7}}$
$=6 \times 10^{-4} m$