A century after Thomas Young’s experiment demonstrated that light behaves as a wave, new discoveries began to challenge that simple picture. Albert Einstein’s explanation of the photoelectric effect showed that light can also behave as a stream of discrete energy packets, or photons. The question arose: is light a wave, or is it made of particles? To answer this, physicists repeated the double-slit experiment under conditions that could test both possibilities.
When light, or even a beam of electrons, is directed toward two narrow slits without being observed, the familiar interference pattern appears on the screen behind them — a clear signature of wave behaviour. However, when the intensity of the source is reduced so that individual photons or electrons pass through the apparatus one at a time, the pattern still slowly builds up over time. Each particle seems to interfere with itself, behaving as if it passes through both slits simultaneously. This strange result suggests that each particle is described by a probability wave, which represents all the possible paths it can take.
The mystery deepens when detectors are placed at the slits to determine which one the particle actually passes through. The very act of measurement destroys the interference pattern, and the results revert to two distinct bands, as expected for particles. It is as though the system “knows” it is being observed and changes its behaviour accordingly. This remarkable outcome reveals the dual nature of quantum objects — they behave as waves when unobserved and as particles when measured.
The double-slit experiment thus became a cornerstone of quantum mechanics. It demonstrated that the classical distinction between particles and waves cannot describe the microscopic world. Instead, light and matter exist in a state of potentiality, represented by probability waves that collapse into specific outcomes upon observation. The experiment challenged the foundations of reality itself, suggesting that the act of measurement is not passive but an active process that helps define what is real.