Electron diffraction is evidence for the wave nature of particles

Energy Levels & Wave-Particle Duality — AQA A-Level Physics

A beam of electrons fired at a thin graphite crystal produces a pattern of concentric rings on a screen behind it.
This ring pattern is identical to what you get when X-rays (waves) diffract through the same crystal.
Diffraction is a wave phenomenon. The fact that electrons produce diffraction patterns proves they have wave properties.
Increasing the electron speed (by increasing the accelerating p.d.) decreases the de Broglie wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m).The wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m). associated with a moving particle, demonstrating wave-particle dualityThe concept that all matter and radiation exhibit both wave-like and particle-like properties. Particles have a de Broglie wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m).; photons exhibit particle behaviour in the photoelectric effectThe emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). is incident on it.... The diffraction rings get smaller because shorter wavelengths diffract less.

Conditions for observable diffraction

Diffraction is most noticeable when the wavelength is comparable to the size of the gap or obstacle.
Atomic spacings in crystals are about $10^{-10}$ m.
Electrons accelerated through ~100 V have de Broglie wavelengths of $\sim 10^{-10}$ m — a good match.
Macroscopic objects (e.g. a tennis ball) have wavelengths of $\sim 10^{-34}$ m. No gap is small enough to diffract them.

Examiner Tips and Tricks

If asked to describe the evidence for wave-particle dualityThe concept that all matter and radiation exhibit both wave-like and particle-like properties. Particles have a de Broglie wavelength; photons exhibit particle behaviour in the photoelectric effectThe emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). is incident on it..: for light, cite the photoelectric effectThe emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). is incident on it. (particle) and diffraction/interference (wave).
For electrons, cite electron diffraction (wave) and deflection in electric/magnetic fields (particle).