In quantum mechanics, the double-slit experiment (often referred to as Young's experiment) demonstrates the inseparability of the wave and particle natures of light and other quantum particles. A coherent light source (e.g., a laser) illuminates a thin plate with two parallel slits cut in it, and the light passing through the slits strikes a screen behind them. The wave nature of light causes the light waves passing through both slits to interfere, creating an interference pattern of bright and dark bands on the screen. However, at the screen, the light is always found to be absorbed as though it were made of discrete particles, called photons.

If the light travels from the source to the screen as particles, then on the basis of a classical reasoning, the number that strike any particular point on the screen is expected to be equal to the sum of those that go through the left slit and those that go through the right slit. In other words, according to classical particle physics 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 unblocking both slits 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, so we know that light must have some particle-wave duality.

Any modification of the apparatus that can determine which slit a photon passes through destroys the interference pattern,illustrating the complementarity principle; that the light can demonstrate both particle and wave characteristics, but not both at the same time. However, an experiment performed in 1987 produced results that demonstrated that 'which-path' information could be obtained without destroying the possibility of interference. This showed the effect of measurements that disturbed the particles in transit to a lesser degree and thereby influenced the interference pattern only to a comparable extent.

The double slit experiment can also be performed (using different apparatus) with particles of matter such as electrons with the same results, demonstrating that they also show