Electron Diffraction
Bragg diffraction 2d sinθ = nλ — Ni crystal lattice + I vs θ plot with n=1,2,3 peaks.
Key Notes
Electrons can diffract off crystal lattices — Davisson and Germer's 1927 experiment was the first direct evidence of matter waves.
An electron beam incident on a single-crystal nickel target produces a diffraction pattern, analogous to X-ray diffraction.
Bragg's law applies: 2d·sin θ = nλ — the same as for X-rays — but here λ = h/p (de Broglie).
G.P. Thomson independently showed electron diffraction through THIN foils, getting Debye-Scherrer rings.
Confirmed de Broglie's hypothesis quantitatively: the measured λ matched h/p for the electron's KE.
Modern applications: electron microscopy (resolution depends on small λ), low-energy electron diffraction (LEED) for surface analysis.
Neutrons also diffract — used in neutron crystallography (better for light atoms like H than X-rays).
Formulas
Bragg's law
d = crystal-plane spacing; θ = glancing angle; n = order; λ = wavelength.
Electron wavelength (NR)
V = accelerating voltage in volts.
Davisson-Germer 54 V check
Matches the measured Ni-crystal diffraction angle precisely.
Important Points
Davisson-Germer (1927) experiment is THE classical confirmation of matter waves and earned the Nobel Prize (1937).
The diffraction pattern from a crystal is sharp because the crystal acts as a 3D grating with periodic d ~ Å.
Higher acceleration voltage ⇒ smaller λ ⇒ smaller diffraction angles (finer pattern).
Electron microscope's resolution is limited by λ — high-V accelerators give λ ~ pm, allowing atomic-scale imaging.
Electron diffraction is destructive testing for biological samples; X-rays are usually preferred for non-conducting crystals.
G.P. Thomson (son of J.J. Thomson, who discovered the electron as a particle) demonstrated its wave nature — father vs son, particle vs wave.
Electron Diffraction notes from sciphylab (also known as SciPhy, SciPhy Lab, SciPhy Labs, Physics Lab). Class 12 physics revision for JEE Mains, JEE Advanced, NEET UG, AP Physics 1/2/C, SAT, and CUET-UG.