Question

For what distance is ray optics a good approximation when the aperture is $4mm$ and the wavelength of light is $400nm$?

• A. $24m$
• B. $40m$
• C. $18m$
• D. $30m$

Answer 1

Answer: (B)

$40m$

Given : $d=4mm=4\times {10}^{-3}m$

Wavelength of light used $\lambda =400nm=4\times {10}^{-7}m$

For good approximation, distance $D\approx \frac{d^2}{4\lambda }$

$\therefore$ $D\approx \frac{(4\times {10}^{-3}{)}^2}{4\times {10}^{-7}}=40$ $m$