# A scientist is investigating the wave and particle properties of electrons. When a beam of electrons is directed at a thin crystal, a pattern of alternating bright and dark rings is observed on a detector screen behind the crystal. The scientist then increases the speed of the electrons in the beam. State what change is observed in the ring pattern and explain, in terms of de Broglie wavelength, why this change occurs.

> WJEC A-Level Physics (Wales) — 3.2 Photon and particle properties · Explain · 5 marks

> Electrons can behave as both particles and waves. The de Broglie wavelength of a particle depends on its momentum.

## Mark scheme (5 marks)

1. The rings become closer together (the pattern becomes more compressed / narrower spacing between rings)
2. Increasing the speed of the electrons increases their momentum
3. A greater momentum means a smaller de Broglie wavelength
4. A smaller wavelength produces less diffraction / the electrons are diffracted through smaller angles
5. Less diffraction results in the rings being closer together on the screen

## Key terms

- [de Broglie wavelength](https://www.gradenine.co.uk/glossary/de-broglie-wavelength)

## Related

- [Revision notes for WJEC A-Level Physics (Wales)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-investigating-the-wave-79ec0456) · Published by Druglandscape Ltd.