Question

What are Fraunhofer lines?

Answer 1

Hint: Fraunhofer lines are actually the diffraction pattern that occurs in the limit of small Fresnel numbers. It is obtained if the source of light and screen are at infinite distance from the obstacle. The Fraunhofer lines have constant intensity.

Complete step by step answer:
Definition:
Diffraction of light is an optical phenomenon in which when a wave is obstructed by an obstacle, the rays bend around a sharp corner this results in formation of beautiful fringe or line patterns comprising maximum and minimum intensity. Depending upon the separation between the source, obstacle and the screen the diffraction are of two kinds (i) Fresnel and (ii) Fraunhofer diffraction.
(i) Fresnel diffraction lines will form when the source of light and the screen are at finite distance from the obstacle.
(ii) The Fraunhofer diffraction line Patterns occur when the source of light and the screen are at finite distance from the obstacle on the spherical surfaces with constant intensity.

In Fraunhofer Plane line diffraction pattern wavefronts are converged by the means of a convex lens to produce a diffraction pattern.

Note: To obtain the Fraunhofer diffraction pattern, the incident wavefront must be plane and the diffracted light should be collected on the screen with the help of a lens. Thus, Fraunhofer diffraction line pattern will form with the help of: (i) Single slit obstacle (ii) Fraunhofer diffraction at circular aperture and (iii) Fraunhofer diffraction at the double slit.