(a) Explain how the intensity of diffraction pattern changes as the order (n) of the diffraction band varies.
(b) Two wavelengths of sodium light 590 nm and 596 nm are used in turn to study the diffraction at a single slit of size 4mm. The distance between the slit and screen is 2m. Calculate the separation between the positions of the first maximum of the diffraction pattern obtained in the two cases.

Explanation

The diffraction pattern due to a single slit consists of a central bright maximum at `O` alingwith alternate secondary minima and maxima on either side. The intensity distribution on the screen is represented in Fig. 6(f).6.
The secondary maxima ate the points in between secondary maxima and are of rapidly decreasing intensity. If `I_(0)` is intensity of central bright maximum, then intensity of first secondary maximum, is found to be `I_(0)//22`, and of second secondary maximum is `I_(0)//61` and so on.
Note that intensity of central maximum is due to wavelets from all parts of the slit exposed to light. In the first secondary maximum, first two parts of the slit send wavelets in opposite phase, which cancel out. Therefore, intensity of 1st secondary maximum is due to wavelets from only one third part of slit. Similarly, the intensity of second secondary maximum is due to wavelets only from one fifth part of the slit, and so on.