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C.3.13 (HL)—Diffraction envelope

Syllabus
First assessment 2025
Objective
—
Level
HL

Read the Diffraction Envelope

HL only

Separate the two patterns

In a multiple-slit intensity pattern, the fine interference maxima are contained within a broad single-slit diffraction envelope. The envelope sets the overall intensity scale; the double-slit or multiple-slit interference determines the rapid fringe structure.

Relate the widths

The single-slit envelope depends on slit width bb, while the separation of fine interference maxima depends on source or slit separation dd. A smaller bb makes the envelope wider; a larger dd makes interference fringes closer together.

Explain missing or unequal maxima

Interference maxima at different angles can have different intensities because the envelope changes across the screen. Some interference maxima may fall at an envelope minimum and disappear.

Common trap

Do not treat every interference maximum as having the same height, and do not confuse the fine fringe spacing with the width of the single-slit envelope.

C.3.13 (HL) Exam Analysis

HL only

Assessment in practice

1–2 marks
How it is assessed

Questions ask you to identify λ/d and λ/b from an intensity graph or explain why different maxima have different heights. The evidence rewards separating the fine structure from the broad envelope.

Command terms

What are / Outline

What earns marks

Identify the broad single-slit envelope separately from the fine interference fringes. Use b for envelope width and d for fringe spacing, then explain unequal or missing maxima as envelope modulation.

Watch for

Attributing unequal maxima only to source brightness and ignoring the single-slit diffraction envelope.

Representative question

Question 1

[Maximum number: 1]

Light of wavelength λ\lambda is incident on two parallel slits of width b that are separated by distance d. The graph of intensity against diffraction angle is shown.

diffraction angle/rad

What are λd\frac{\lambda}{d} and λb\frac{\lambda}{b} ?

λd\frac{\lambda}{d}

λb\frac{\lambda}{b}

0.1

0.1

0.1

0.01

0.01

0.1

0.01

0.01

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