Question

What is the effective distance between the source of light and the screen in Fraunhofer Diffraction?

(a) Focal length of the convex lens

(b) Less than Focal Length of the convex lens

(c) Greater than the focal length of the convex lens and less than infinite

(d) Infinite

I have been asked this question in homework.

My question comes from Fraunhofer Diffraction in portion Optics of Engineering Physics II

Answer 1

The correct option is (d) Infinite

Best explanation: In Fraunhofer Diffraction, the source of light and the screen are effectively placed at infinite distance. Two convex lenses are used for achieving such a condition. Thus, the incident waveform is plane and the secondary wavelets are in the same phase at every point in the plane of the aperture.