Explain the diffraction of light at a single slit. Why is the central maximum the brightest, and why do the secondary maxima get fainter? 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 · Explain the central maximum. At the centre, wavelets from matching points of the slit arrive in phase, so it is brightest.
3. 3 · Explain the minima. Where the edge path difference is λ, the two halves of the slit cancel in pairs (λ/2 apart), so it is dark; minima lie at a sin θ = nλ.
4. 4 · Explain the secondary maxima. Where the edge path difference is 3λ/2, two thirds of the slit cancel and one third adds up, so the maximum is much fainter.
5. 5 · Draw the graph. [Intensity against θ: tall, broad central peak; zeros at a sin θ = ±λ and ±2λ; smaller humps getting fainter outward]
6. 6 · Real life. A tube light seen through a narrow gap between two fingers shows faint bands.
7. 7 · Close in one line. So, a single slit gives a broad bright centre with fainter bands on each side.

Scoring tip: Mark the zeros on the θ axis and label the central maximum; show it twice as wide as the others.