Write the expression for the width of the central maximum in single slit diffraction. State how it depends on slit width and wavelength, and give two differences between interference and diffraction patterns.

1. 1 · Write the result. The central maximum lies between the first minima at ±λ/a: angular width 2λ/a, linear width 2λD/a.
2. 2 · Slit width. Width ∝ 1/a: a narrower slit gives a wider central maximum.
3. 3 · Wavelength. Width ∝ λ: red light gives a wider maximum than blue; in water it narrows, as λ becomes λ/μ.
4. 4 · Sketch the pattern. [Intensity graph with the central peak between −λ/a and +λ/a; width marked 2λ/a]
5. 5 · Two differences.
   • Interference: equal fringe widths; diffraction: central maximum twice as wide
   • Interference: bright fringes about equally bright; diffraction: brightness falls quickly
6. 6 · Close in one line. So, the narrower the slit, the more the light spreads.

Scoring tip: This line is qualitative: give the formula 2λD/a and the proportions, with no derivation.