Double-Slit Interference of Light

As was first described by Thomas Young in 1801, if a monochromatic light source illuminates a plate pierced by two parallel slits, and the light passing through the slits is observed on a screen located behind the plate, then bright and dark bands appear on the screen. This experiment is usually thought of as a demonstration that light is a wavelike phenomenon. In fact, the conventional explanation is that incident light waves pass through both slits, and then travel slightly different distances to a given point on the screen, where they interfere with one another to produce a bright band if the path difference is an integer multiple of a wavelength, and a dark band if the path difference is a half-integer multiple of a wavelength.

How do we account for double-slit interference at the individual photon level? In fact, in 1909, Geoffrey I. Taylor showed that an interference pattern is generated in a double-slit experiment even when the incident light intensity is so low that only a single photon could be in the apparatus at a given time. The only way in which to account for this result is to assume that an individual photon incident on the apparatus passes though both slits, and then interferes with itself when it reaches the screen. In other words, a photon in the apparatus is partly in a state in which it passed through one slit, and partly in a state in which it passed through the other. Moreover, the interference between these two states at the screen can only determine the probability of the photon being observed at a given point on the screen.