Explain why the fringes in a double-slit interference pattern become less bright and eventually disappear at large angles from the centre of the pattern.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a double-slit experiment, monochromatic light passes through two narrow slits separated by a small distance. A screen placed at a large distance from the slits shows a pattern of alternating bright and dark fringes.
Model answer (4 marks)
Each slit alone gives a single‑slit diffraction pattern with a bright central maximum that falls off with increasing angle.
The interference pattern from the two slits is superimposed on this diffraction envelope – the interference fringes are modulated by the single‑slit intensity.
As the observation angle increases, the single‑slit intensity decreases, so the bright interference fringes become progressively dimmer.
At the angles where the single‑slit diffraction has minima, the intensity is zero, so the corresponding interference fringes disappear (missing orders).
The interference pattern from the two slits is superimposed on this diffraction envelope – the interference fringes are modulated by the single‑slit intensity.
As the observation angle increases, the single‑slit intensity decreases, so the bright interference fringes become progressively dimmer.
At the angles where the single‑slit diffraction has minima, the intensity is zero, so the corresponding interference fringes disappear (missing orders).
Examiner tips
- Use the word ‘modulated’ or ‘envelope’ to link diffraction and interference.
- Show the relationship between single‑slit intensity and fringe visibility.
- Mention missing orders at diffraction minima.
Common mistakes
- Confusing diffraction with interference; not linking the envelope to the fringes.
- Saying fringes become dimmer but not that they vanish at diffraction minima.
- Using vague terms like ‘weaker’ without explaining the cause.
Mark scheme (4 marks)
- Each individual slit produces a single-slit diffraction pattern, which has a bright central maximum that decreases in intensity with increasing angle.
- The double-slit interference fringes are modulated by (sit within) this single-slit diffraction envelope.
- As the angle increases, the single-slit diffraction intensity decreases, so the bright interference fringes become progressively dimmer.
- At the minima of the single-slit diffraction pattern the intensity falls to zero, causing entire interference fringes to disappear (missing orders).
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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