Question

The radius of the first zone in a zone plate of focal length 40 cm for light of wavelength 4000 Å is:

• A. 0.4 cm
• B. 0.04 cm
• C. 0.16 cm
• D. 0.010 cm

Hint:

The first zone radius in a zone plate is calculated using \( r_n = \sqrt{n f \lambda} \).

Answer 1

The Correct Option is B

The radius of the first zone in a zone plate is given by:
\[ r_n = \sqrt{n f \lambda} \] For the first zone \( n = 1 \), \( f = 40 \) cm, and \( \lambda = 4000 \) Å \( = 4 \times 10^{-5} \) cm:
\[ r_1 = \sqrt{(1)(40)(4 \times 10^{-5})} \] \[ r_1 = \sqrt{1.6 \times 10^{-3}} = 0.04 \text{ cm} \]