Wave-Particle Duality

We all know light has some properties. Some of its properties are reflection, refraction, dispersion, total internal reflection, diffraction, interference and polarisation, which means that since light is a transverse wave, the direction of propagation of the wave is perpendicular to the direction of movement of the particles.

Light also has a dual nature. That means that depending on the phenomenon or behaviour in question, light can be taken as a wave or as a particle.


The Dutch physicist Christiaan Huygens first provided evidence that light was a wave, in the 1600s. But he was countered by Sir Isaac Newton in the late 1600s, who suggested that light was actually made of tiny particles, which were almost like atoms, which he called ‘corpuscles’. But if it were made of particles, when you cross 2 light beams, some interaction should take place. Think of each light beam as being made of tiny marbles. If you crossed their paths, some of the marbles would collide with each other, and bounce off randomly, giving almost “showers” of light. But that isn’t the case with light. The light beams just pass through one another, so it must be a wave. As stated at the beginning, one of the properties of light is interference. Interference patterns are the complicated shapes (or ripples) that can be seen when two wave patterns occupy the same space. It’s like if you drop two pencils that are very close to each other into a pond, the ripples will overlap. The same phenomenon can be seen when two point-like sources of light are placed near each other, so light has to be a wave, because particles cannot make interference patterns. Thomas Young also conducted an experiment called the Double-Slit Experiment in the early 1800s, which provided practical proof that light was a wave. In this experiment, Young used a screen microscopic slit, and passed a light beam through that. The light beam diffracted and when these diffracted beams passed through two other slits, they appeared on the screen as multiple beams of light, not just two (which would have been the case if light was a particle). The individual slits acted as individual sources, which light passed through and spread. This proved that light was a wave, and not a particle, since diffraction is only a property of waves. So light has to be a wave, right? Well, not necessarily only a wave. If you shine a torch onto a sheet of metal, the metal heats up, because light is transferring energy to the atoms in the metal in discrete (individually separate and distinct) packets known as ‘quanta’.* Philipp Lenard, a German physicist in the 1900s, also conducted an experiment and discovered that when light of specific frequencies was shined on a metal, it basically “knocked off” some electrons. He called this the ‘photoelectric effect’. He also observed that different colours (essentially different wavelengths of light) were responsible for this phenomenon. Shorter wavelengths, like blue or violet light, produced electrons with greater amounts of energy, and the number of ejected electrons was directly related to the intensity of the light shined. So a brighter light emitted more photons. Light was behaving like a particle here, because it was “kicking off” electrons, and unlike what is seen with waves, an increase in the brightness of the light didn’t increase the energy of the ejected electrons. This provided counter-evidence for the previous wave theory. Albert Einstein provided evidence to prove the same particle-like properties of light, and finally physicists decided it made sense. They called this property the dual nature of light. This idea that light sometimes behaved as a particle and sometimes as wave led to the revolutionary new physics theory called quantum mechanics. Wave-particle duality is the concept in quantum mechanics that every particle may be partly described not only in terms of particles, but also in terms of waves. So all the particles we know- electrons and atoms included- are also waves. For macroscopic particles, wave properties are usually harder to detect because of their extremely short wavelengths, but for most microscopic particles, quantum mechanics seems to be able to explain that all particles can also act as waves. *A quantum (plural: quanta) is the minimum amount of any physical entity involved in an interaction. So a photon is a single quantum of light (or any other form of electromagnetic radiation.) Sources: https://www.youtube.com/watch?v=J1yIApZtLos https://www.slideshare.net/teacherfidel/10-properties-of-light https://www.youtube.com/watch?v=gkNPopdxViY www.wikipedia.org

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