10.10 Estimate the distance for which ray optics is a good approximation for an aperture of 4 mm and wavelength 400 nm.

Hint: Fresnel’s distance () is the distance that is used in ray optics for a good approximation.

Step 1: Find Fresnel’s distance ().

ZF=a2λ
Where,
Aperture width, a = 4 mm = 4 × 10-3 m
Wavelength of light,  = 400 nm = 400 × 10-9 m

ZF=(4×103)2400×109=40 m

Therefore, 40 m is the distance for which the ray optics is a good approximation.