Question
Explain diffraction by a single slit and write down the conditions for central maximum, secondary maxima and minima.
✓
Answer
→ When the double slit in Young's experiment is replaced by a single narrow slit (illuminated by a monochromatic source), a broad pattern with a central bright region is seen.
→ On both the sides, there are alternate dark and bright regions, the intensity becoming weaker away from the centre. (Fig. (b))
→Fig (a) shows a parallel beam of light falling normally, on a single slit LN of width $a$. The diffracted light goes on to meet a screeen. The midpoint of the slit is M .
→A straight line through M perpendicular to the slit plane meets the screen at $c$.
→We want the intensity at any point P on the screen. As before, straight lines joining P to different points $L, M, N$, etc. can be treated as parallel, making an angle $\theta$ with the normal MC.
→Here, the basic idea is to divide the slit into much smaller parts and add their contributions at P with the proper phase difference.
→We are treating different parts of the wavefront at the slit as secondary sources. Because the incoming wavefront is parallel to the plane of the slit, these sources are in phase.
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