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C.3.12 (HL)—Single-slit diffraction

Syllabus
First assessment 2025
Objective
—
Level
HL

Model Single-Slit Diffraction

HL only

Recognize the pattern

A monochromatic wave passing through a narrow rectangular slit spreads and forms a broad central maximum with weaker side maxima separated by minima. The pattern results from interference between contributions across the slit.

Use the first-minimum condition

For slit width bb, the first minimum satisfies θ≈λ/b\theta\approx\lambda/b for small angles. On a screen a distance xx away, the central maximum width is approximately 2xλ/b2x\lambda/b.

Check the trends

A narrower slit or longer wavelength produces greater angular spreading and a wider central maximum. A wider slit or shorter wavelength produces a narrower pattern. The syllabus treatment is monochromatic light and rectangular slits at normal incidence.

Common trap

Do not confuse the distance from the central maximum to the first minimum with the full central-maximum width; the latter is twice the first-minimum distance on the screen.

C.3.12 (HL) Exam Analysis

HL only

[ 2 ]

The graph shows the variation with diffraction angle θ\theta of the intensity I on the screen.

\(I / \mathrm{Wm

The slit width is 1.3×10−5 m1.3 \times 10^{-5} \mathrm{~m}. Calculate the wavelength of the light.

Retrieve the HL C.3 Wave Phenomena Model

HL only

The HL extension is secure when you can model diffraction as interference and read the limits of the pattern.

  • Single-slit diffraction: b sinθ = nλ for minima
  • Diffraction envelopes in multiple-slit patterns
  • Diffraction grating maxima: nλ=d sinθ
  • Allowed orders satisfy nλ≤d