Sponsored links

Ok, Dear we now consider fraunhoffer diffraction of monochromatic light by a narrow slit. Let a parallel beam of monochromatic light be incident normally on an opaque plate having a long narrow slit in it.

According to geometrical optics, the transmitted beam, on the screen, should have uniform illumination of the same cross-section  (X,Y) as that of the slit. Contrary to this, when a transmitted beam is focused by a lens on the screen, we observe a diffraction pattern. It consists of
(i) A very intense central bright band, much wider than the slit width, in the direction of incidence.

ii)  A set of dark and subsidary maxima of decreasing intensity, on either side of central bright band.

iii) The central bright maxima is called Central Maxima and secondary Maxima.
To obtain a Fraunhoffer class of diffraction due to a slit, the incident wavefront is plane which is obtained by using  a convex lens in such a way that source lies in the focal plane of the lens. The diffracted light from the slit is again collected on the screen with the help of convex lens, so that the source and screen are at infinite distances from the obstacle. The experimental arrangement is a source of monochromatic light of wavelength.