The speed of light in different media

You've probably heard the phrase "Nothing is faster than light" or "Light is the speed limit of the universe." Light is energy, in the form of electromagnetic waves, your monitor screen produces the colored visible light that you need to read this post, but the WiFi signal that transmits from your home router to your computer is also light, it is invisible and comes in the form of radio waves called microwaves.

All these forms of light are electromagnetic radiation and they vibrate, the visible light that is at the center of this diagram vibrates with a frequency that can stimulate the cells of our eyes so that we perceive it in the form of colors; What is to the right of visible light in the diagram below, ultraviolet light, x-rays and gamma rays, are forms of light that vibrate faster, so their wavelengths are short and cause cancer.

And those that are to the left of visible light, infrared light, microwaves and radio waves vibrate more slowly, so their wavelengths are longer and do not cause cancer, that's right, neither the signal from your phone, nor the 5G cell towers, nor your microwave oven are going to give you cancer.

All these forms of light are vibrations in the electromagnetic field that propagate in the form of waves, like waves in the water of a pond and the speed at which these waves travel is the speed of light, but the speed of light is not the fastest there is, in fact, light can go at such different speeds that taking it as a reference is relative.

This number 299,792,458 meters per second is important, we also know its rounded version of 300 million meters or 300,000 km per second, which is what I was taught in school was the speed of light, what the first unrounded number actually represents is the speed limit of the universe, nothing can go faster than 299,792,458 m/s, at least not in the universe that we know.

This figure is one of the fundamental constants of physics, this number is super practical because it does not have decimals and represents the speed of light traveling through a vacuum. In theory, this sounds like light is the fastest thing in the universe, but the funny thing is that this would only occur in an absolute vacuum and that simply does not exist, at least not in the universe we know.

Even the most empty regions of matter among galaxy clusters have atoms, they are generally hydrogen and they are scattered here and there in very low concentrations, but they are there. The vacuum of space is not actually empty, so light traveling through there will never reach this speed, it comes very close, but it does not arrive, in any case, this figure is very useful because it helps us understand a lot of phenomena in our universe.

To avoid writing this number every time a physicist wants to refer to the speed of light in a vacuum, simply write the letter c, some believe that it is for the Latin word for speed celeritas or simply for the c for constant, whatever, if you write lowercase italic c in something in physics, it is understood that you are referring to the speed of light in a vacuum, but that light going at the maximum speed of the universe, we already know that it cannot happen because there are no true vacuums.

Even if we could remove every atom from a container, which is impossible to achieve, a phenomenon called quantum fluctuation would magically make virtual particles and photons appear and disappear in that vacuum, so it wouldn't really be empty, yes, quantum is magic, black magic, but then, if light doesn't travel at the maximum speed of the universe, better known as c, how fast does it travel? That depends on what kind of light and through what.

Light in space is very fast, light from the sun traveling to Earth, even though it encounters much more matter than in interstellar space, almost travels at the speed of c, but when light enters the atmosphere, it encounters air, air is a transparent medium, it is also called electric, which although it allows light to propagate through it, makes it move slower.

Light waves retain their frequency, but their propagation speed decreases. The higher the frequency of light, the greater its interaction with the medium and the slower it travels through it. When light travels through air, its speed decreases to 899.911 m/s. That is, its speed is 99.97% of the constant c and there are other media that, although they are still transparent or dielectric like air, are much denser and light travels much slower through them.

For example, light travels through water at only 75% of its speed in the Vacuum, the light that carries the internet signal to your home through fiber optics travels at 66% and through a diamond it travels at 40%, that is less than half the speed it has in space, so it all depends on the type of light and through which that light circulates.

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