This article explains thin-film interference and shows how an everyday optical coating reveals the strange behavior of light.
An anti-reflective coating is a very thin film applied to glass to reduce reflection. This technology is now widely used, and can even be found on high-end wristwatches.
Why can such a thin film suppress reflections? The answer is both fascinating and counterintuitive.
Light can behave like a stream of particles. However, if we think of light only as particles traveling along definite paths, it becomes difficult to explain why a thin film can reduce reflection.
The key is not that the surface is perfectly smooth, but that the film has just the right thickness. When this condition is met, the light reflected from the front surface of the film and the light reflected from the back surface cancel each other out, greatly reducing the amount of reflected light. This phenomenon is called interference.
The particles of light are called photons, and in quantum mechanics even a single photon can interfere with itself. In that case, should we say that the photon was reflected from the front surface, or from the back surface? Surprisingly, the interference still appears even when photons are sent one at a time.
For a photon to interfere with itself at the same place and time, it seems as if it must have taken more than one path, since light always travels at the speed of light. Did the photon really travel along multiple paths? Here the mystery becomes even deeper.
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