X-Ray Diffraction Laue Photographs
X-rays can be individually counted by a Geiger Counter, but they also exhibit interference and diffraction.
A beam of x-rays is diffracted by a thin crystal in a set of directions.
Each direction is a reflection from a plane of atoms.
This experiment also shows crystals are a regular array of atoms.
Notes:
X-rays were discovered by Roentgen a little over a century ago, and they turned out to be very high frequency electromagnetic radiation, or photons. Since they caused individual clicks in a Geiger counter, they seemed to be localized, like particles, but when a beam of x-rays were directed at a crystal, they diffracted like waves to form a diffraction pattern on a photographic film. The only way to explain that was to suppose they were waves that interacted simultaneously with many atoms at once.