By Mohamed Aziz Chebil

Wave - Particle duality

The question we will answer today is “What is Light? Is it a wave or is it a particle?”

To answer this question, we will get back in time and see the history of scientists who tried to solve this problem.

We start with Isaac Newton who proposed that light was formed by a group of particles in the 17th century. This is known as the Corpuscular Theory.

At the same time, Christian Huygens proposed that light is a form of a wave.

Due to the popularity of Newton, most scientists believe Newton’s theory was the right one.


In 1801, a scientist named Thomas Young published a paper entitled “On the theory of light and color” where he questioned Newton’s theory and confirmed Huygens’s theory using the double slit experiment.

Using that experiment, Young proved that light can behave like a wave.

On the far side of the divider, the light from each slit diffracts and overlaps with the light from the other slit, interfering with each other.

The interference pattern is unique for waves like sound waves or water across a body of water. Since there is an interference pattern using light, we can conclude that light is a wave and not a particle as Newton suggested.

Yet, this is not everything. Scientists were able to prove that light also behaves like a particle.

Before we get to that, let us understand the particle pattern in the double-slit experiment. You will see that particles will end up in a line mostly in the same place depending on the angle the particle passed through.

We will take into consideration that light is made of photons. By using photons instead of grains of sand we can carry out the double-slit experiment on an atomic scale. If you block one of the slits and fire protons to the sensor screen, you will see that the photons will end up in a line mostly in the same place the same pattern as the grains of sand. We could suggest that photons are particles.

Now if you launch the photons through the double slits, something strange will happen: we notice an interference pattern.

This unique behavior of photons is an indicator of the wave-particle duality nature
of photons.
The same nature applies to electrons which behave both as a particle and wave at the same time.

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