Question

If the Fresnel’s distance is a, then what should be the distance of the screen from the slit, d, such that ray optics is no longer valid?

(a) d > a

(b) d < a

(c) d = a

(d) No relation between d and a

The question was posed to me in an international level competition.

The doubt is from Diffraction of Light in portion Diffraction of Engineering Physics II

Answer 1

The correct answer is (b) d < a

The explanation is: As long as d < a, the concepts of ray optics cannot be used directly. The distance at which the spreading of light due to diffraction becomes equal to the size of the slit is known as Fresnel’s distance.