Explain how the Davisson–Germer experiment provided evidence for the wave nature of electrons.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In 1927, Davisson and Germer directed a beam of electrons at a nickel crystal and detected a strong reflected intensity at specific angles. The pattern they observed was consistent with that expected from wave interference.
Model answer (4 marks)
The electrons were reflected from the nickel crystal and produced a diffraction (interference) pattern with intensity maxima at specific angles. Such a diffraction pattern can only arise if the electrons behave as waves, with adjacent atomic planes in the crystal acting as a diffraction grating (similar to X‑ray diffraction). The de Broglie wavelength of the electrons (λ = h/p) matched the wavelength calculated from the angular positions of the diffraction maxima using the Bragg condition / constructive interference condition. This confirmed de Broglie’s hypothesis that matter (particles with mass) possesses wave‑like properties and demonstrated wave–particle duality for electrons.
Examiner tips
- Use the term ‘diffraction pattern’ and ‘intensity maxima’ to show understanding of wave interference.
- Explain that only waves can produce such a pattern, linking to Bragg’s law.
- Show the calculation of λ from the experiment and compare with λ = h/p.
- Mention wave–particle duality explicitly to hit the final point.
Common mistakes
- Confusing constructive interference with destructive interference; students may say ‘no pattern’ instead of ‘diffraction pattern’.
- Failing to mention the Bragg condition or the de Broglie wavelength calculation.
- Using the wrong formula for λ (e.g. λ = h/mv without recognising p = mv).
Mark scheme (4 marks)
- Electrons were reflected from the nickel crystal and produced a diffraction (interference) pattern with intensity maxima at specific angles.
- Such a diffraction pattern can only arise if the electrons behave as waves, with adjacent atomic planes in the crystal acting as a diffraction grating (similar to X-ray diffraction).
- The de Broglie wavelength of the electrons (λ = h/p) matched the wavelength calculated from the angular positions of the diffraction maxima using the Bragg condition / constructive interference condition.
- This confirmed de Broglie's hypothesis that matter (particles with mass) possesses wave-like properties and demonstrated wave–particle duality for electrons.
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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