# Explain why two coherent sources of waves are necessary to observe a stable interference pattern.

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

> Two loudspeakers are connected to the same signal generator and placed a small distance apart. A student walking along a line parallel to the loudspeakers notices alternating regions of loud and quiet sound.

## Mark scheme (4 marks)

1. Coherent sources maintain a constant phase difference (which may be zero) between the two waves.
2. For constructive interference to occur, the path difference from the two sources to a given point must equal a whole number of wavelengths (nλ), resulting in a maximum.
3. For destructive interference to occur, the path difference must equal a half-integer number of wavelengths ((n + ½)λ), resulting in a minimum.
4. If the sources were not coherent, the phase difference would vary randomly over time, so the positions of maxima and minima would shift continuously and no stable pattern would be observed.

## Key terms

- [coherent sources](https://www.gradenine.co.uk/glossary/coherent-sources)
- [stable interference pattern](https://www.gradenine.co.uk/glossary/stable-interference-pattern)

## 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-two-coherent-sources-of-3727901d) · Published by Druglandscape Ltd.