# 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.

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

> 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.

## Mark scheme (4 marks)

1. Each individual slit produces a single-slit diffraction pattern, which has a bright central maximum that decreases in intensity with increasing angle.
2. The double-slit interference fringes are modulated by (sit within) this single-slit diffraction envelope.
3. As the angle increases, the single-slit diffraction intensity decreases, so the bright interference fringes become progressively dimmer.
4. At the minima of the single-slit diffraction pattern the intensity falls to zero, causing entire interference fringes to disappear (missing orders).

## Key terms

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

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-fringes-in-a-010247cf) · Published by Druglandscape Ltd.