Question

For what distance is ray optics a good approximation when the aperture is 4 cm and wavelength is 600 nm?

• A. 4.2 km
• B. 2.7 km
• C. 1.8 km
• D. 1.2 km

Answer 1

The correct option is B 2.7 km

Fresnel distance = $\frac{a^2}{\lambda }$
$=\frac{4\times 4\times {10}^{-4}}{600\times {10}^{-9}}=\frac{16\times {10}^{-4}}{6\times {10}^{-7}}$
$=\frac{16}{6}\times {10}^3$
$=\frac{8}{3}$ km
= 2.7 km