How does light slow down?

The phenomenon of light slowing down as it passes through a material like glass or air is one of the most fascinating areas of physics, involving a complex interaction between light and materials. There are three ways to look at the same situation, and each employs a different understanding of physics.

All of these explanations have strengths and weaknesses, but all of them are powerful tools for understanding this fascinating interaction.

 Related: How does astronomy use the electromagnetic spectrum?

View No. 1: It’s all waves

The first perspective comes from James Clerk Maxwell, the 19th-century Scottish physicist and all-around genius who discovered a unified theory of electricity and magnetism, and also found that light is made of waves of electricity and magnetism.

When these waves encounter a material like glass or water, they see a whole bunch of charged particles. The molecules in the material are made of atoms, which have protons and electrons — both charged particles. And charged particles respond to electromagnetic waves passing by them by wiggling along with them.

But moving charged particles also create electromagnetic waves of their own. The result is a giant mess, with the original electromagnetic waves interfering with all the waves generated by all the charged particles in the material (and there are a lot). Thankfully, most of those waves, except the waves traveling in the original direction of the light, cancel each other out. But because the waves generated by the particles are a little delayed, the entire ensemble moves more slowly.

The end result: The light moves more slowly.

View No. 2: It’s all particles

Maxwell’s theory is a classical picture of radiation. Nowadays, we have a much more sophisticated view based on quantum mechanics, where light is made of countless tiny particles known as photons. Photons can act individually, but when enough of them get together, they display all of the same properties as electromagnetic waves.

A fully quantum treatment of photons interacting with materials can get pretty nasty, but thankfully, we have an approach developed by the famed physicist Richard Feynman to guide us through it. We can imagine all the photons of the incoming light slamming into the material. Once inside, they begin interacting with all the charged particles.

Those charged particles can absorb those photons and emit their own, because that’s what charged particles do. But these photons are a little different. In physics, they’re known as virtual photons.

Remember that photons can do two things: They can roam freely through the universe, existing as independent entities (this is light), and they do the legwork of mediating the electromagnetic force (like the force holding a magnet to a fridge). But these photons don’t roam freely; they have a job to do. So we call them “virtual” photons — they exist only in our math to help us account for the electromagnetic force.

So all of these charged particles start emitting copious amounts of virtual particles, and once again, there’s a giant, confusing mess. Feynman came to the rescue. He developed a technique of averaging out all of the possible paths that those photons can take. That averaging process eliminated all the wayward photons, leaving behind only the ones traveling in the original direction of the light. But all of those interactions come at a cost: It takes time for an electron to absorb and reemit a photon, and those delays add up.

The end result: The light moves more slowly.

View No. 3: It’s all polaritons

So far, we’ve focused on the properties of light, viewing it through a particle-based lens and a wave-based lens. But the material is more than a simple collection of charged particles that just do whatever they are electromagnetically ordered to do.

Materials can be interesting, too. Specifically, all materials can support vibrations — little ones, big ones, ones that last a long time, ones that fade away quickly. All material is constantly in motion, and that motion affects how that material interacts with everything else. To help physicists grapple with the complexities of all the kinds of vibrations that are constantly racing through materials, they proposed an entity known as a phonon.

A phonon is another kind of fake particle, but like virtual photons, it’s very useful. It allows physicists to use the language of quantum mechanics to describe the vibrations in a material. This new language comes in handy when light, which is made of photons, enters that material.

When photons and phonons get together, they create something new: a polariton. In this view, when light enters a material, it disappears. And so do the phonons in the material itself. Instead, they get replaced by polaritons. These polaritons share a lot of properties with their parents, but they have one crucial property: They travel more slowly than the speed of light.

That speed depends on the properties of the material (the phonons). In this view, it’s not light that’s passing through a material, with the material responding to it, but a new object, a polariton, passing through. This view is especially useful, because in many situations, it’s very easy to discard all the cumbersome math of conflicting waves or bouncing photons and just deal with a straightforward, simple entity that already encodes all the information you need.

Light goes in, a polariton travels through and light goes out.

The end result: The light moves more slowly.

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