light SEEMS TO move at different speeds outside of a vacuum

In a vacuum, light moves at a constant speed of 300 000 km/s (abbreviated to a lowercase c). It has zero mass, and thusly moves at the fastest possible speed, also known as the cosmic speed limit. Maxwell’s Equations taught us the speed of light is constant and invariant to all observers. It is a cornerstone of how our reality works.

Ole Rømer is the guy credited for proving that light, at all, has speed and isn’t instant. In 1676 (starting a small decade earlier), he compared the times of the eclipses of Io (a moon of Jupiter) at different distances from Earth. He noticed that when Earth was further away from Jupiter, the time between Io’s eclipses became longer. This time difference was measured to around eleven minutes. Not trivial. He deduced light has a speed, as it took longer to arrive to Earth the further away Io was (to be fair, Christiaan Huygens is the one that calculated c - but he did so on Rømer’s data).

However, and despite the fact that light always travels at a constant speed, we have found it slows down when it travels through a non-vacuum, such as water, glass, diamonds, or even air. This is why we see images bend when it goes through the water. Like the straws in the above image. It is also why glasses and binoculars, at all, work. We can manipulate light, so it fits optimally to your eyes. It is also why diamonds glitter so uniquely. The light inside of diamonds have slowed down 50%, giving off this mesmerizing and expensive shine.

So, from a certain point of view, it would seem the speed of light is not so constant? When in a vacuum it is precisely c, and always c. When through water it is c-25%, and always c-25%, etc. It means that once it exits a glass of water, it will instantly change its speed to that of air, or glass of whichever is the other medium. No acceleration or deceleration is occurring. There is only a change of states. From 100% to 99%, to 75% and back to 100%.

Light can even stop completely in its travel, and continue thereafter again. Think in a supercooled cloud of crystals. Look up experiments spearheaded by Professor Lene Hau of Harvard University. She successfully slowed down the speed of light to less than a speeding car, and later she managed to stopped it completely, only to release it afterwards.

The above illustration shows the different speeds light has, through the respective media. We call this the refractive index. However, why is it so? Why does light slow down, and how does it instantly speed up again? You will find many explanations to this on the internet, and many of them are wrong.

the wrong explanations

You will probably meet explanations saying light merely travels a longer path as it is interrupted by particles of matter. Water, for example, is denser than air, and naturally much denser than a vacuum. Light particles will bump into atoms as it shines through, and thusly it will be delayed in its track depending on how many atoms(degree of density) there is — a bit like how you are delayed if you have to pass through a room full of people. You won’t be able to move in a straight line, and you will have to maneuver around the many people. Once you leave the room, you are again moving at your original straight-line speed.

Sounds sensible, and you will find this analogue in many textbooks, but it is false. True for people, not for light particles (aka photons). For one, it invites a traveled path that may vary - both in speed and lengths, but also in where the light particle may exit. This would make glasses, for example, an impossibility. In the refractive index, light particles act exactly as predicted every time. We know how fast, and where it will exit. With this knowledge, we can make glasses. There is no stochastic pattern in the refractive index, and, therefore, this explanation must be rejected.

Another popular explanation is that light gets absorbed by atoms in a medium, and is then momentarily released later to continue on its path. This process will indeed delay the light, and would seem to explain what we observe. It doesn’t. Though atoms do absorb light, this won’t explain the exactitude of the refractive index. For one, such absorption is particular to specific wavelengths and not all wavelengths, plus it too follows a stochastic pattern - which we know, the refractive index never does. Alas, this is also false.

The video below, from a very popular YouTube channel, is an example of an incorrect explanation. It sounds sensible, but it does not correspond to our knowledge of what really goes on. This is a nice way of saying that the “educational” video below is spreading misinformation.

the right explanations

The surprising fact is that the individual photons do actually not slow down in water, glass, or other media. They keep their constant speed of 300 000 km/s, even when apparently slowed down. The change in speed is only apparent, not factual (textbooks usually do say this, but the last sentence here is often missed or forgotten).

And so, we have the first answer to the popular question on how light can instantly speeds up after leaving a medium like water; it never speeds down. The speed of light (or rather, the speed of light particles) is constant even when it appears not to be.

The deeper explanation to this peculiarity, unfortunately, is not easy to comprehend or to explain. It is remarkably complicated, which is why there are so many more or less inaccurate attempts to explain it out there.

Anyway, here goes my attempt:

The refractive index of light is 25% slower when it moves through, for example, water. Where in a vacuum, it moves at a 100% constant c.

Two different approaches can explain this phenomenon. Both are correct, although one is more correct than the other.

• The classical explanation views light as a wave. When the wave enters the medium, it will oscillate or tickle surrounding atoms. Each of these atoms will begin to produce electromagnetic waves on their own via all the oscillating electrons. A chaotic dance party happens, and all the waves bouncing among the atoms, will excite more and more. Add all these waves up, and we will end with a refractive index, exactly as predicted.

This accumulation of excited waves will give the appearance of a 25 % decrease in light speed. Waves can either cancel other waves out, or they can build bigger waves. In other words; the net effect of the light coming in, and all the electromagnetic waves created by the reacting atoms within the medium, will add up to what we measure as a 25% slower speed. One could say that the light waves are not themselves changed. Still, they are affected by these other electromagnetic waves inside the medium, making the accumulated wavelength shorter.

Although this explanation does hold up mathematically and fully corresponds with our equations and theories on the subject, the quantum mechanical explanation below paints a slightly different picture.

• The quantum mechanical explanation views light as wave-functions. You can read about wave-functions here. We say the photon-wave-function goes into the medium and will go through every possible path in this medium - even absurdly circular motions or what else we can probabilistically calculate. This subatomic behavior is often described as quantum superpositions - a particle has all positions, not just one. Absurd at this sounds, it corresponds with experiments. Sounds similar to the dance party of the waves, as described above, but quantum mechanically (and mathematically) it is not. The final light of all these superpositions is then a 25% reduction. Precisely as measured.

Which one of the two models you prefer is up to you. They are both equally right within their paradigm. Although similar, know that the quantum mechanical model is superior when we go into the details of many other facets of the refractive index of light.

There are some other models still, which see the photon inside a medium as a different type-particle. The medium is thusly considered a completely different system. Often this non-photon is called a Polariton, and since this holds mass, it incidentally means it travels at less than c. To the best of my understanding, it is a kind of a mix of the two above. The model satisfies some peculiarities but is insofar not verified beyond its mathematics.

... I told you. It is remarkably complicated.

To sum it up (a bit): Light moves at a constant speed. When it enters different media, like glass or water, the quantum mechanical wave-function will dictate all possible states for this light, which, depending on the medium, will influence the passing of light particles so their apparent speed changes.

Check out Professor Michael Merrifield of the University of Nottingham. The 16-minute clip is well worth your time on this subject:

And also Professor Philip Moriarty of the University of Nottingham:

And this too from Adjunct Professor Don Lincoln of the University of Notre Dame:

Photos via Google