Young's Two-slit Experiment

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The two-slit experiment had, and still preserves a fundamental impact on modern physics. In his Lectures on Physics, Feynman stated that the interference patterns seen in Young’s experiment are “a phenomenon which is impossible, absolutely impossible to explain in any classical way, and which has in it the heart of quantum mechanics. In reality it contains the only mystery.” [1] Here, he is expressing the wonder of wave-particle duality, in which matter at an atomic level behaves at times like a particle and at others, like a wave. However, observations of both qualities at the same time are impermissible for some reason by the laws of quantum mechanics.

Within the two-slit experiment, there are numerous aspects that defy logic and maintain the mystery that is quantum physics. The experiment itself works by emitting photons of light from a monochromatic source to pass through a single slit to obtain a narrow beam of light. This narrow beam is then allowed to fall on a screen containing two narrow slits. Light from the two slits then falls on a distant screen. The resulting patterns of light on the screen can then be observed. What Young found was that a central bright maximum of light was obtained, and around this central bright maximum were equidistant striations of light and dark. To clarify one point, it was later discovered that if a single photon were fired at the two slits, an interference pattern would not appear. Rather, a single 'blip' would appear on the screen, which indicates that the photon is not a wave, but rather a particle. If a large number of photons were fired at the slits, an interference pattern would begin to appear. So it seems that photons are really particles that behave collectively like waves. The same reasoning applies to all particles, not just photons.

Many variations of the Young two-slit experiment have since arisen, namely gedankens (thought experiments), to illustrate the fundamental princi...