# Explain why the pattern of maxima and minima observed in single-slit diffraction differs from that observed in double-slit interference, even when the slit separation and wavelength are the same.

> IB DP Physics Higher Level (2023 syllabus) — C.3 Wave phenomena · Explain · 4 marks

## Mark scheme (4 marks)

1. In double-slit interference, the two slits act as point (coherent) sources, producing equally spaced maxima of equal intensity.
2. In single-slit diffraction, path differences arise between wavelets from different parts of the same slit (Huygens' wavelets / secondary sources across the slit width), causing the maxima to have decreasing intensity away from the centre.
3. The central maximum in single-slit diffraction is twice as wide as the secondary maxima (minima occur at angles where the path difference across the full slit equals a whole number of wavelengths).
4. In the double-slit pattern the single-slit diffraction envelope modulates the intensity of the interference fringes, so fringes near the edges of the envelope are dimmer and some interference maxima may be missing where they coincide with single-slit minima.

## Key terms

- [diffraction](https://www.gradenine.co.uk/glossary/diffraction)
- [interference](https://www.gradenine.co.uk/glossary/interference)

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-pattern-of-maxima-d7bdaabc) · Published by Druglandscape Ltd.