Page "Double-slit experiment" Paragraph 1

In the basic version of the experiment, a coherent light source such as a laser beam illuminates a thin plate pierced by two parallel slits, and the light passing through the slits is observed on a screen behind the plate.

The wave nature of light causes the light waves passing through the two slits to interfere, producing bright and dark bands on the screen — a result that would not be expected if light consisted strictly of particles.

However, on the screen, the light is always found to be absorbed as though it were composed of discrete particles or photons.