# Explain why a single narrow slit produces a diffraction pattern with a bright central maximum flanked by alternating dark and bright fringes of decreasing intensity.

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

> Monochromatic light of wavelength 550 nm is directed at a single narrow slit. A screen placed far from the slit shows a diffraction pattern consisting of a bright central maximum and several secondary maxima of diminishing brightness on either side.

## Mark scheme (4 marks)

1. Each point on the wavefront within the slit acts as a source of secondary wavelets (Huygens' principle), and these wavelets superpose / interfere beyond the slit.
2. The central maximum is bright because wavelets from across the entire slit arrive at the centre of the screen with zero path difference, so they interfere constructively.
3. Dark fringes (minima) occur at angles where wavelets from different parts of the slit can be paired so that each pair has a path difference of half a wavelength, resulting in complete destructive interference.
4. Secondary maxima are less bright than the central maximum because only a fraction of the slit's wavelets contribute constructively (the remaining wavelets cancel), and the fraction contributing constructively decreases for higher-order maxima.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-single-narrow-slit-8174e99f) · Published by Druglandscape Ltd.