# Explain why increasing the number of slits in a diffraction grating, while keeping the slit separation constant, produces sharper and brighter principal maxima.

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

> A diffraction grating consists of a large number of equally spaced slits. When monochromatic light passes through it, a pattern of sharp, bright maxima is observed at angles that satisfy the grating equation d sin θ = nλ.

## Mark scheme (4 marks)

1. Each additional slit contributes an additional wave/wavelet that can interfere constructively at a principal maximum, so the amplitude (and hence intensity) at those maxima increases.
2. Between principal maxima, waves from the many slits have a range of path differences and therefore destructively interfere / cancel more completely.
3. This more complete destructive interference in the regions between maxima means the bright maxima are confined to a narrower angular range, making them sharper / narrower.
4. The positions of the principal maxima are unchanged because they depend only on the slit separation d and wavelength λ (via d sin θ = nλ), not on the number of slits.

## Key terms

- [diffraction grating](https://www.gradenine.co.uk/glossary/diffraction-grating)
- [slit separation](https://www.gradenine.co.uk/glossary/slit-separation)

## 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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