The double-slit experiment is an experiment that demonstrates the wave nature of light.

A coherent light source illuminates a thin plate with two parallel slits cut in it, and the light passing through the slits strikes a screen behind them.

If light is a particle, then only the rays of light that hit exactly where the slits are will be able to pass through and will make a pattern of two exact lines, like the slits, on the viewing screen.

If light is a wave, then the wave nature of light causes the light waves passing through both slits to diffract and overlap and as a result an interference pattern of bright and dark bands will be seen on the screen. And this interference pattern is actually seen on the screen, and by this is demonstrated the wave nature of light.

The wave nature of light can be also demonstrated in another way by the "double slit experiment". If the light travels from the source to the screen as particles, then the number of particles that strike any particular point on the screen should be equal to the sum of those that come from the left slit and those that come from the right slit. In other words, the brightness at any point should be the sum of the brightness when the right slit is blocked and the brightness when the left slit is blocked. However, it is found that blocking one slit makes some points on the screen brighter, and other points darker. This can only be explained by the alternately additive and subtractive interference of waves, not the exclusively additive nature of particles.

The double-slit experiment is generally thought to have been first performed by the English scientist Thomas Young in the year 1801 in an attempt to resolve the question of whether light was composed of particles (Newton's "corpuscular" theory), or rather consisted of waves traveling through some ether, just as sound waves travel in air (as the theory of Dutch physicist Christiaan Huygens). The interference patterns observed in the experiment seemed to discredit the corpuscular theory, and the wave theory of light remained well accepted until the early 20th century, when evidence began to accumulate that light, which exists in tiny "packets" called photons, exhibits properties of both waves and particles (Einstein's photoelectric effect and Planck's research). This property is referred to as the wave–particle duality of light.