It’s a well-known fact that light slows down in water or glass, or any other transparent medium. Even more interestingly, after leaving that medium, it goes back to its original speed. Yes, it speeds up! But, how could that happen? Why does light slow down in water or glass, and why and how does it increase its speed once it left the medium? Where does that extra energy come which speeds it up again?
In the video published by the Fermilab channel below, Dr. Don Lincoln explains why.
The speed of light in a vacuum, commonly denoted c, is a universal physical constant. It is always 299,792,458 meters per second (which is about 300,000 kilometers per second or 186,411 miles per second) regardless of whether the source was moving relative to the observer or not.
If you shoot a laser at the glass, the light from the laser will travel at c, until it hits the surface of the glass. Then it will slow down to about 70 percent of its original speed (about 2/3 c), while also changing its direction. Once the light gets to the other side of the glass, it will emerge from the glass, change its direction again, and move again at c. How that can happen?
Dr. Lincoln first talks about two common but wrong answers to this question and then gives the correct answer.
Wrong answer 1:
The first completely wrong answer to why does light slow down in a medium is the idea that light scatters off atoms as it passes through them.
According to this explanation, the light hits an atom, goes careening of an angle, then hits another atom, so on… Light always travels at c between the atoms, but the distance it covers increases, so it appears as it is slowed down.
The problem is… this “explanation” cannot explain many things.
First of all, scattering isn’t a precise process. Light can scatter in lots of directions. We should see different speeds each time we repeat the test because the distance that the light covers would be slightly different each time. But, we always measure the same speed.
Furthermore, there is no way to guarantee that the light would end up traveling in the original direction. In other words, the light wouldn’t follow the path we observe. Instead, it would come out of the medium with a range of velocities and a range of angles.
Wrong answer 2:
The second wrong idea about why the light slows down in a medium is that light is not scattered, but rather that it is absorbed and re-emitted by the atoms. Again, between the atoms, the light travels at c.
The amount of time the atom takes to absorb the light and re-emit it seems to slow the light.
The problem here is that when absorbs a photon, it doesn’t remember where the photon was coming from. So, when it emits the photon, it can do it in any direction. In this case, the light would scatter even more. Some photons should even return back! But we don’t observe that.
So, this very common explanation that photons are absorbed by atoms and then re-emitted which causes the delay that we observed is also plain wrong.
So, why does light slow down in water or glass? And how can it speed up again when it leaves the medium?
Because light is also a wave. The light slows down in a medium like glass or water, and this phenomenon can be understood considering light as an electromagnetic wave, more than a quantum particle.
When an electromagnetic wave (light is an electromagnetic wave) enters a medium, this incoming wave (w1) starts interacting with the electrons surrounding the atoms of the medium. This interaction creates an oscillation of the electrons in the medium (with the same frequency as w1) which, in turn, generates a second electromagnetic wave (w2) within the medium.
In the medium, you now have a new “master wave” (w3), resulting from the sum of the incoming wave (w1) with the second wave (w2) of the medium. It is that new master wave, that is different from the incoming wave, that moves at a slower speed. It is as if the generated wave (w2), when summing the crests and troughs of the 2 waves (w1 and w2), creates an apparent delay for w3.
When the light (w1) leaves that medium, there’s no more a second wave (w2), there’s no summing of the waves, and as a result, there’s no master wave (w3). There’s only w1. The light continues its journey in the same direction at its original speed, c.
Wait, why that master wave (w3) goes slower?
In the video above, Dr. Lincoln beautifully explains this.
We can “add” two waves together, and it’s easy. You just take the height of two waves and add them together. If they are lined up so the peaks are at the same place, the result is a single wave with a higher peak.
If two equal waves (both have the same wavelength and wave height) are lined up so a peak corresponds to a trough, they cancel each other.
If these two waves have different wavelengths, you end up in a funny-looking shape.
It gets more interesting when one wave is moving at a different speed than the other one. The result is a different wave, but one that has a different speed than either of the two. Its speed will be in-between.
Sources
- Snell’s law on Wikipedia
- Wave-particle duality on Wikipedia
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