Why does light slow down in water?

The Internet is a very powerful tool that puts humanity's knowledge at our fingertips. However, the Internet is also a source of misinformation. You can hear all kinds of explanations, expressed with authority and with good intentions, but also completely and totally wrong. So in this video, I thought I would address a common one and give a correct explanation for why

light

slow

s down in

water

. Actually, that's just a catchy title. I want to tell you why

light

slow

s down in any transparent medium: certainly

water

, but also glass, plastic and even air. If you've ever taken a physics class, you learned about the refractive index of a material.

The refractive index is just a fancy term for how much light is slowed down in that material. The refractive index, always written as n, is a number greater than one. And the speed of light in a material is simply the speed of light in a vacuum, divided by the refractive index. I put a table here to give you an idea of ​​how much light can be slowed down by common materials. This slowdown is the reason objects appear bent when stuck in water. It's because the path the light follows curves when it enters or leaves the water.

It's the reason you can get images like this here. And I should let you know that no physics teachers were injured taking that photo. There are a lot of interesting phenomena I could show you that arise due to the refractive index and the slowing down of light, but I want to focus entirely on the slowing down. So to do that, let's look at this animation. Suppose you have a piece of glass with a refractive index equal to 1.5. That means light will travel at 2/3 the speed of light in the glass. If you shoot a laser at glass, the laser light will travel at the speed of light until it reaches the surface of the glass.

When it hits the surface of the glass, the light will slow to 2/3 of its original speed and change direction. Once the light reaches the other side of the glass, it will emerge from the glass, change direction, and move again at the speed of light. That is what happens. Furthermore, the light emerging from the glass will travel in exactly the same direction; well, more precisely, in a path parallel to the direction of the incident light. The path is shifted a little to the left, but the angles are the same. So that's what happens. It's been tested and there's no debate about any of it.

Now let's move on to the real question. Why

does

light slow down? And how can he speed up again when he leaves? There are two very common explanations that you find on the Internet and both are completely wrong. Let me tell you first. The first one makes some sense. Remember it's wrong, but it makes sense. It is the idea that light is scattered in atoms as it passes through glass. Light hits one atom and is bent at one angle, then hits another atom and is bent at a different angle. Light always travels at the speed of light between atoms.

That process happens over and over again as light passes through the glass, but let's focus just on these two scatterings to see how the explanation goes. If we remove the atoms, we can see the important bits. There is the actual path taken by the light in this scenario, which we can compare with the path in a straight line. If we straighten the real path, we see that it is longer than the straight path. And, if light travels at the speed of light in a vacuum between scatterings, we see that it will take longer than if it traveled the distance in a straight line.

Taking more time is another way of saying, effectively, go slower. And this is how some people explain the lower effective speed of light in glass. So how do we know it's wrong? Well, dispersion is not a precise process. Light can be scattered in many directions. For example, it could have been dispersed as we see here. This is much more severe dispersion, with a much longer path, and consequently the light would be much slower if this happened. Furthermore, there is no way to guarantee that the light will end up traveling in the original direction. In fact, if light did what is claimed, it would not follow the path we observe.

Instead, light would come out of the glass at a variety of speeds and a variety of angles. This idea simply

does

n't work because it doesn't make correct predictions. So, it's not right. The second misconception is that light is not scattered, but rather absorbed and re-emitted by atoms. Between atoms, light travels at the speed of light in a vacuum. The amount of time it takes for the atom to absorb and emit light appears to slow down the light. You can see what I mean in this example here. The speed of the moving photons is always the same, but the time they take during absorption and emission makes the solid line take longer than the dashed one.

This effectively slows down the light. Now, this idea, although sensible, is also erroneous. Let's see why. The problem is that when an atom has been absorbed by a photon, it does not remember where the photon came from. And when it emits a photon, it can do so in any direction. So what would actually happen in the absorption/emission case is something more like this. The photon would be absorbed and re-emitted in a direction desired or not. In the end, the passage of light through the material would end up looking like this, well, that's not what we see.

So if none of these ideas work, which one does? First, I must say that these two ideas have sort of buried deep down the idea that light is a particle. However, the reason why such light slows down in matter is best illustrated if one accepts the idea that it is a wave. There are many properties of a wave, the most important being the wavelength. The wave oscillates up and down, with a separation between peaks and a time between oscillations. A second important property is what we call superposition, which is just a fancy way of saying that you can add them together.

And adding them is easy. You simply take the height of the two waves and add them together. If the two waves are aligned so that the peaks are in the same place, the result is a single wave with a higher peak. If the wave is aligned so that a peak corresponds to a trough, then the two waves cancel out. And, if one wave has a different wavelength than the other, the result will be a strange shape. Is that how it works. It becomes more interesting when one wave moves at a different speed than the other. The result is a different wave, but one that has a different speed than either of the two.

This is very important, so let's take a closer look at it. If we add the two upper waves, which have different speeds, the lower wave also moves, but more slowly. So let's go back to light moving through glass or water. Remember that light is a wave of electric fields. It oscillates with a characteristic wavelength and frequency. That wavelength depends on the color of the light, with blue light being about 400 nanometers and red light being about 700. The numbers don't matter as much as remembering that light changes electric fields. Of course, glass is made of atoms, which are surrounded by electrons.

Electrons have an electrical charge and that charge feels a force from the oscillating field of light. Because it feels a force, the electrons move. But moving electric charges also create their own oscillating electric field. Simply put, the oscillating electric field of light causes the electrons to move, which creates a second oscillating electric field. And, if you have two oscillating electric fields, they are two oscillating waves and you can add them together as we saw before. The net effect is that the two waves combine and form a single wave of oscillating electric fields. And that is the wave that moves through matter.

And, and this is important, it moves slower than light in a vacuum. This also explains something that confused many people. Most people were willing to accept that passing through glass would slow down the light. But they were very puzzled by the fact that the light accelerated when it left the glass. It seemed like that created energy and, well, it didn't make any sense. But now I hope so. Before light reaches the glass, it travels at the speed of light in a vacuum. When it passes through the glass, the light causes the electrons to move and generate a second wave that adds to the light.

Light still moves at the speed of light in a vacuum, but the wave of electrons moves at a different speed and the combined wave moves more slowly than light would if the atoms were not there. Then when the light leaves the glass, there are no atoms around to create electric fields to add to the light and then the light moves forward at the preferred vacuum speed. That's all. Light effectively travels slower through the material because when it is in the material it generates a second wave that combines with the light and the new combined wave moves slower than the familiar speed of light.

Ta da! Alright, tell everyone. This is something you often hear erroneous explanations about on the Internet. But now you know and that means you are eligible to be a member of the physics kids club. Well, that was pretty amazing. I hope you remembered to like, subscribe and share. I always love finding out the underlying answer to how things work, especially when you often find incorrect explanations online. But, as is often true, the way to find the right answer is to learn some physics, because, well, as I'm sure you'll agree, physics is everything.