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Wave-Particle Duality: An Intro to Quantum Mechanics

By Matthew H on February 2, 2024

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Quantum mechanics1^1

is a large and complex field that; much like the other branches of physics, one could spend days if not weeks pouring over just to gain just a cursory understanding. However, unlike kinematics or electrostatics, quantum mechanics emerged relatively recently, in the early 20th century.

This new branch of physics represents a paradigm shift from classical physics. While classical mechanics concerns itself with the macroscopic2^2

, quantum mechanics deals with the microscopic 3^3

. More importantly, quantum mechanics relies on probabilistic equations and uncertainty as opposed to the deterministic equations and definite outcomes that define classical mechanics. To put it simply, when we look at very small particles, things get weird.

One such phenomenon is wave-particle duality, a fundamental concept in quantum physics that describes particles exhibiting both wave-like and particle-like properties. This brief overview will focus on specific experiments that demonstrate light's dual nature.

Firstly, if we want to prove that light exhibits wave-like properties, we can look no further than the double-slit experiment. Originally conducted by scientist Thomas Young, this experiment involves a light source, a barrier with two slits, and a detection screen placed behind the barrier. When light passes through the slits, it creates an interference pattern on the screen, characterized by alternating light and dark bands.

So, how does this experiment demonstrate light exhibiting wave-like behavior? Well, the alternating light and dark bands, or more accurately the interference pattern, is a hallmark of wave behavior. The only way to achieve these distinct, alternating bands of light is for the light to be propagating from the two slits, each as a wave toward the screen. To explain why this is true, we need to know about the superposition principle for waves which is explained in a footnote4^4

. Now that we know the superposition principle, the light bands are areas of constructive interference where the troughs5^5

of each wave meet the troughs of the other and the crests of one meet the crests of the other. Knowing what we know about superposition, light waves “adds up” and create a band of light. As for the areas with less light, destructive interference is at play. Destructive interference occurs when crest6^6

meets trough and vice versa, and light waves “cancel out” to make the dark spots. We know that the interference pattern that light is making is of a wave-like behavior because we observe this behavior in other sources that propagate in waves. Think of sound or actual waves, you will notice that they also exhibit interference patterns.

However, that’s only half the story because light also exhibits particle-like behavior. A bit over a century after the double-slit experiment, Albert Einstein came up with an explanation for a phenomenon that had eluded scientists since the late 1800s: the photoelectric effect. Simply put, the photoelectric effect describes the emission of electrons by, in most cases, metals under certain frequencies of light. However, the wave theory of light could not explain why certain frequencies of light would never induce the photoelectric effect. If light was a wave, increasing the amplitude of the wave would cause energy delivery to increase, and yet, extremely intense high-amplitude light waves of low frequencies would not induce the photoelectric effect while low-amplitude light waves of high frequencies would at least cause a few electrons to be emitted.

To explain this discrepancy, Einstein theorized that light did indeed behave like a particle: it was “quantized” into certain extremely small units (particles) of light called “photons.” The energy of a photon was determined by the frequency of the light, and Einstein used this new theory to explain that the energy of a single photon was what determined if the photoelectric effect would be induced. To use an analogy for this explanation, it is as if the photons can be considered attacks and the material that it is hitting has an inherent “defense” attribute to it. The defense of the material, which would be analogous to the work function of the material, would completely negate the attacks of the photons if they were not high enough, and the material would not take any “damage.” Only photons with a high enough attack, or energy, would induce the photoelectric effect.

And so, with that, we have shown how light can exhibit both wave and particle-like behavior. Wave-particle duality is just one of the several interesting phenomena in the realm of quantum mechanics. If you want to learn more, make sure to check out the physics sessions on

Thank you Hafsah M editing this article!

  1. In this article, I use quantum mechanics and quantum physics interchangeably
  2. Macroscopic: Observable by the naked eye; for our intents: relatively big things
  3. Microscopic: Unobservable by the naked eye, requiring the use of a microscope; for our intents: very small
  4. In this case, when two waves meet the resulting displacement of the waves will be the sum of the individual displacements
  5. Lowest point in the cycle of a wave
  6. Highest point in the cycle of a wave

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