Explain the diffraction of light at a single slit. Show how the central maximum and the first minimum arise, and draw the intensity distribution.

1. 1 · Define diffraction. Bending of light round edges into the geometrical shadow, caused by wavelets from different parts of one wavefront.
2. 2 · Describe the set-up. A plane wavefront falls normally on a slit of width a; the pattern is seen on a distant screen.
3. 3 · Explain the central maximum. At θ = 0, all wavelets are in phase, so the centre is brightest.
4. 4 · Explain the first minimum. Edge-to-edge path difference is a sin θ. At a sin θ = λ, the two halves cancel in pairs (path difference λ/2), so dark. Minima lie at θ = nλ/a.
5. 5 · Explain the secondary maxima. At aθ = 3λ/2, two thirds cancel and one third remains: a weak maximum. Maxima lie near (n + ½)λ/a and fade outward.
6. 6 · Draw the graph. [Intensity against θ: tall central peak; zeros at ±λ/a and ±2λ/a; smaller humps decreasing outward]
7. 7 · Real life. A tube light seen through a narrow gap between fingers shows faint bands.
8. 8 · Close in one line. So, a single slit gives a broad bright centre with fainter bands on each side.

Scoring tip: Mark λ/a and 2λ/a on the θ axis of the graph, and label the central maximum.