Electron Diffraction and Davisson-Germer Experiment
In the Davisson-Germer experiment, accelerated electrons scatter from a crystal and produce intensity maxima described by Bragg's law. Agreement between the inferred wavelength and h/p confirms the de Broglie hypothesis.
Why this shows up in the exam
Crystal-structure measurements · Low-energy electron diffraction · Experimental verification of matter waves
Learn the idea
Electron diffraction verifies matter waves by matching crystal interference to the de Broglie wavelength. A crystal provides regularly spaced atomic planes like a three-dimensional diffraction grating. Peaks appear only when electron waves scattered from neighboring planes return in phase.
🧠 Memory hook: Crystal peak wavelength must agree with h over p.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- n lambda = 2 d sin(theta_B) — Bragg condition for planes separated by d at Bragg angle theta_B
- lambda = h/sqrt(2 m_e e V) — non-relativistic electron wavelength for comparison with diffraction
How to approach it
- 1Identify the geometric angle used in Bragg's law
- 2Compute electron wavelength from the accelerating voltage
- 3Match the Bragg and de Broglie wavelengths with consistent units
Common slip-ups that cost marks
- •Using the angle between incident and reflected beams directly as the Bragg angle
- •Forgetting the diffraction order n
- •Calling electron diffraction proof of electron charge rather than wave behavior
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Photons of energy 5 eV illuminate a metal of work function 2 eV. Find the stopping potential.
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