The double slit experiment is a demonstration that matter and energy can display characteristics of both waves and particles and demonstrate the fundamentally probabilistic nature of quantum mechanical phenomena.
In this experiment, a light source, such as a flash light illuminates a thin plate pierced by two parallel slits. 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. This result would not be expected if light consisted only of particles. However, even when there is an interference pattern the light is always found to be absorbed at the screen as though it were composed of discrete particles or photons. This result shows the principle of wave-particle duality.
If light was only ordinary or classical particles and these particles were fired in a straight line through a flit and allowed to strike a screen on the other side, we would expect to observe a pattern corresponding to the size and shape of the slit. However, when this sing slit experiment is performed, the pattern on the screen is a diffraction pattern in which the light is spread out; the smaller the slit, the greater the angle of spread.