The Real Reason Why Light Is Slower in Water
By Jon Therkildsen, MSc MBA from University of Århus (2004)
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 thus moves at the fastest possible speed, also known as the cosmic speed limit. Maxwell’s Equations taught us that 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 his work 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 be around eleven minutes. Not trivial. He deduced light has a speed, as it took longer to arrive at Earth the further away Io was (to be fair, Christiaan Huygens is the one who calculated c - but he did so on Rømer’s data).
However, 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 they pass through water. Like the straws in the above image. It is also why glasses and binoculars, at all, work. We can manipulate light so that it fits your eyes optimally. It is also why diamonds glitter so uniquely. The light inside diamonds has slowed by 50%, giving them this mesmerizing, 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, glass, or whichever other medium it enters. 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 path and continue thereafter. Think of 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 stop it completely, only to release it afterward.
Wavelengths and Speed of light in Different Media
The above illustration shows the different speeds at which light travels 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 for this on the internet, and many of them are wrong.
the wrong explanations
You will probably encounter explanations saying that light merely travels a longer path because 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 they shine through, and thusly they will be delayed in their track depending on how many atoms(degree of density) there are, 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 analogy 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 length, 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 is absorbed by atoms in a medium and is then re-emitted, continuing along 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 specific to certain wavelengths, not all, and it also 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 align with our understanding of what really goes on. This is a nice way of saying that the “educational” video below is spreading misinformation.
1:41 minutes. I love minutephysics on YouTube, but this video is plain wrong.
the right explanations
The surprising fact is that the individual photons do not actually slow down in water, glass, or other media. They maintain a constant speed of 300 000 km/s, even when they appear to be 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 speed up after leaving a medium like water; it never slows down. The speed of light (or rather, the speed of light particles) is constant even when it appears not to be.
The deeper explanation for 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 reduced by 25% when it passes 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," the 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 out other waves or 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 while the frequency remains unchanged.
Although this explanation holds up mathematically and fully aligns 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 a wave function. 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 as this sounds, it corresponds with experiments. Sounds similar to the dance party of the waves described above, but quantum-mechanically (and mathematically) it is not. The final light of all these superpositions is then a 25% reduction of the refractive index. 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 examine the many other facets of the refractive index of light in detail.
There are some other models still, which see the photon inside a medium as a different type-particle. The medium is thus 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 sort 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, such as glass or water, the quantum-mechanical wave function determines all possible states of the light, which, depending on the medium, influence the propagation of light particles, causing their apparent speed to change.
Check out Professor Michael Merrifield of the University of Nottingham. The 16-minute clip is well worth your time on this subject:
16:26 minutes. Sixty Symbols, YouTube
And also Professor Philip Moriarty of the University of Nottingham:
9:50 minutes. Sixty Symbols, YouTube
And this too from Adjunct Professor Don Lincoln of the University of Notre Dame:
10:23 minutes. Fermilab, YouTube.
Photos via Google