Question

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

Answer 1

The distance for which the ray optics is a good approximation is known as the Fresnel’s distance.

Fresnel’s distance is given by,

Z F = a 2 λ

Where, Z F is the Fresnel’s distance, a is the aperture width and λ is the wavelength.

Aperture width is given as 4 mm=4× 10 −3  m.

Wavelength of the light is given as λ=400 nm=100× 10 −9  m.

Substitute the values in the above formula.

Z F = ( 400× 10 −3 ) 2 400× 10 −9 =40 m

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