Is Light a Particle or a Wave?

In 1672 a debate began which continued for 180 years. It centred on the fundamental question: what is the nature of light? On one side of the argument, we have Isaac Newton and his corpuscular (particle-like) theory. While on the other, there was Christiaan Huygens and his wave theory of light. But who was right? Somehow… they both were!

Newton’s Corpuscular Theory

So, let’s begin with Newton. In the year this all started, 1672, Newton published the ‘New Theory of Light and Colour’. This set out a few of his key principles about light. The most important of these was that light is made up of seven different colours, and that each colour was a unique (‘tennis-ball like’) particle. 32 years later, in 1704, he published another book called ‘Optiks’ which described his new comprehensive theory of light. His main postulate was that the light ‘particles’ he theorised previously are referred to as ‘corpuscles’ and are tiny, massless particles that only travel in straight paths.

The concept of corpuscles was not invented by Newton, but it was certainly developed mostly by him. ‘Corpuscularianism’ has similar roots to ‘atomism’, which was the idea that everything was made up of tiny, indivisible particles called ‘atomos’ (now known as atoms). The key difference with corpuscles is that they are not necessarily indivisible. Regardless, the key takeaway is that Newton believed light travelled as a stream of particles.

There were several other postulates. Firstly, he believed that corpuscles travel through transparent media at very high speeds in all directions. He thought they entered and interacted with our eyes (causing the sensation of vision), where different colours arose from difference sizes of corpuscles. Finally, they are repelled by reflective surfaces and attracted by transparent surfaces.

This theory was popular for several reasons. One was because of Newton’s great prestige, which is also a large reason why his beliefs took precedence over Huygens’ to begin with. But the other is because it was able to explain several light-related phenomena. For example, the observation of sharp shadows could be explained by the fact that light travels in straight lines as discrete particles, and when an opaque object gets in the way, they are blocked. Any particles which are not blocked will carry on as normal. The (unfortunately blurry) image below demonstrates this nicely.

Combining the theory with Newton’s famous laws of motion was also able to explain reflection and refraction. It is clear that light reflects off certain materials, but why? Why must it reverse its direction as opposed to just passing through? Well as he noted in his theory, because the particles have changed direction, their velocity has changed direction, and thus a force must have acted on the them. So he believed reflection was due to a force that pushed the particles away from the surface. He thought this force acted perpendicular the surface, hence the component of the velocity parallel to the material is unchanged, while the component perpendicular to the material is reversed.

As for refraction, which is the observation that light changes speed (and direction) when moving through a different medium, he proposed that there is a force of attraction between matter and the corpuscles, causing the velocity component perpendicular to the material to increase, giving rise to the change in direction. The denser the medium, the more matter there is, meaning the force is greater and so a larger refraction.

Although this explains the path tending tending towards the perpendicular line, it implies that the speed of light has increased. More generally, it describes that the denser a medium, the faster light travels through it. We know today that this is wrong — it slows down. Furthermore, there are a few other things the theory couldn’t explain. That’s where the wave theory comes in.

Huygens’ Wave Theory

The notion of light being a particle could not explain interference, polarisation, and most notably: diffraction. Interestingly, this phenomena was first discovered by an English physicist, Robert Hooke, in 1672 — the same year that Newton published his first particle theory of light.

Diffraction is the reason you can hear someone from around a wall without actually being able to see them. This makes sense for sound, as we know it is a longitudinal wave. But Hooke was able to show that light also undergoes this phenomena. This was later reinforced by Thomas Young in the famous ‘Young’s double slit experiment’. With the advancement of engineering and technology, scientists were able to create diffraction gratings which show the diffraction of light even more clearly.

Hopefully you can see that this contradicts the idea that light only travels in straight paths. As a result, in 1678, Christiaan Huygens hypothesised that light is a longitudinal wave. Most people eventually abandoned Newton’s corpuscular theory in favour of Huygens’ wave theory for not only did it explain diffraction, but it was still able to explain most other affects like refraction.

Given that light is a wave, it has a wavelength (the distance between two adjacent peaks). He predicted that when light enters a denser medium, its wavelength decreases. Given that the speed and wavelength of a wave are proportional, this means the speed also decreases when it enters a denser medium — which is correct. If the light is entering at an angle, the portion of the wave in the medium changes speed, while the portion outside remains constant. This leads to the ‘bend’ in the path.

While this theory was certainly able to explain more than Newton’s, it still couldn’t explain sharp shadows… which Newton’s could! Clearly neither theory was perfect, and elements of both are correct depending on the situation. It’s almost as though light can act as both a wave AND a particle. Well, that’s what we believe today!

Wave-particle duality

We now enter the realm of quantum mechanics, which gets vastly more complicated and unintuitive. Soon, I’m going to do a blog post along the lines of ‘Quantum Mechanics In A Nutshell’, so stick around for that! But for now, the key point is that light can propagate forward like a wave, but sometimes show the characteristics of a particle (hence ‘wave-particle duality’).

This can change depending on several factors, one of which is the frequency of the wave. Frequency and wavelength are inversely proportional, so as frequency increases, wavelength decreases. Given the amount of diffraction that occurs increases with wavelength, a higher frequency means less diffraction. In other words, the shorter the wavelength, the less ‘wave-like’ the light is. Furthermore, this idea of wave-particle duality also applies to particles too. An electron, a typical particle, can undergo a wave phenomena like diffraction. As I said, there’s a lot of very weird quantum mechanics going on, and even whether you’re looking at the experiment can change its outcome!

So, what is the conclusion of this debate? Everyone is both right and wrong at the same time. Not sure if that’s a particularly satisfying ending or not, but that’s the universe we live in!

Interested in this topic? Watch the video below to learn more.

Originally published at http://thephysicsfootprint.com on February 3, 2022.