Light: The Quantum Quandary

Light. It is essential for our existence and we take it for granted. But what is it? We cannot feel it, but we can manipulate it using mirrors and lenses. It definitely travels as it takes time for light to reach us from distant cosmic bodies but how does it travel? Could light be materialistic in nature and consist of discrete particles or is it a wave (like sound)?

Well… light has in fact been shown to take the form of either photons (small “packets” of energy) or an electromagnetic wave, but never both simultaneously. This phenomenon is known as the principle of complementarity.

So light can act as a wave one minute and a particle the next? This does seem to be the case. Allow me to elaborate…

Light as a Wave

In Young’s experiment or the “Double-slit” experiment, light can be seen to act as a wave. In Young’s experiment, light was passed through two slits, and rather than seeing two illuminated bands on the subsequent detection screen (what you might expect to see for discrete particles), multiple bands of alternating maximum and minimum intensities with graded intermediate intensities were observed, indicative of “interference”. Interference is a characteristic of waves, in which two waves superpose such that two peaks will summate to form a larger peak whereas a peak and a trough will effectively cancel each other out.


The single-slit and double-slit experiments in water. Waves originate from a point and are diffracted as they pass through either a single-slit or a double slit. In the double-slit experiment, the two resultant waves superpose to produce the characteristic interference pattern. Yellow arrows are indicative of points of maximum detected wave amplitude.

Light as a Particle

So, Young’s experiment suggests that light is a wave, which may be diffracted in much the same way that sound or water waves are. However, acting solely as a wave cannot explain other observations of light. If light is shone on a piece of metal, electrons are emitted. This is due to the light transferring energy to the electrons within the metal. When light is incident upon a metallic surface, a resident electron may gain enough energy to be liberated from the metal and be emitted as a “photoelectron”.  This is known as the photoelectric effect and was explained by Einstein in 1905 by defining light as consisting of quantum-sized packets of energy (photons) with an energy equal to the frequency (f) of the observed light wave multiplied by a constant, the Plank constant (h). This would explain why electrons are not eventually emitted from a metallic surface when illuminated by a dim light. Rather incident light must be of a threshold frequency. Electrons absorb photons and are elevated to a higher energy level and if the absorbed photon is of sufficient energy (E=hf) the electron may be emitted from the surface. This is an all or nothing event. If light were to act as a wave in this circumstance, it would be logical to think an electron may accumulate energy over a duration of light exposure but this just isn’t the case.

So what is it…?

Both of these models for light have been accepted as an over-simplification for something that no-one really has a definite answer to.

Next time:

Making light work of protein structure analysis


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