It’s a well-known fact that light slows down in water, glass, or any other transparent medium. Even more interestingly, it returns to its original speed after leaving that medium. Yes, it speeds up! But how can that happen? Why does light slow down in water or glass, and how does it increase its speed once it leaves the medium? Where does the extra energy come from to speed it up again?
In the video published by the Fermilab channel below, Dr. Don Lincoln perfectly explains why.
The speed of light in a vacuum, commonly denoted as c, is a universal physical constant. It is always 299,792,458 meters per second (about 300,000 kilometers per second or 186,282 miles per second), regardless of whether the source is moving relative to the observer or not.
If you shoot a laser at 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 (around 2/3 c), while also changing its direction. Once the light exits the glass, it will change direction again and resume traveling at c. How can that happen?
Dr. Lincoln first discusses two common but incorrect answers to this question before providing the correct explanation.
Why does light slow down in water or glass? Wrong answer 1:
The first completely wrong answer to why light slows 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, scatters at an angle, then hits another atom, and so on. Light always travels at c between the atoms, but the distance it covers increases, making it appear as if it has slowed down.
The problem is that this “explanation” cannot account for many things.
First of all, scattering isn’t a precise process. Light can scatter in many directions. We should see different speeds each time we repeat the test because the distance the light covers would be slightly different each time. However, 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 emerge from the medium with a range of velocities and a range of angles.
Why does light slow down in water or glass? Wrong answer 2:
The second incorrect idea about why light slows down in a medium is that light is not scattered, but rather 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 an atom absorbs a photon, it doesn’t remember from which direction the photon was coming from. So, when it emits the photon, it can do so in any direction. In this case, the light would scatter even more. Some photons might even return back! But we don’t observe that.
So, this very common explanation that photons are absorbed by atoms and then re-emitted, causing the delay we observe, is also plainly wrong.
So, why does light slow down in water or glass? And how can it speed up again when it leaves the medium? The true explanation
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 does the master wave (w3) go 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.
This combined wave results in an apparent delay, creating the effect of light slowing down. Once the light exits the medium, the secondary wave disappears, allowing the light to resume its original speed, c.
In conclusion, the phenomenon of light slowing down in a medium like glass or water can be understood by considering light as an electromagnetic wave. When light enters a medium, the incoming wave interacts with the electrons of the medium, creating a secondary wave. The combination of these waves forms a new master wave that moves at a slower speed, due to the interaction and summation of their crests and troughs. Once the light exits the medium, the secondary wave disappears, and the light resumes its original speed. This process highlights the wave nature of light and explains why it slows down in different transparent mediums.
Sources
- Snell’s law on Wikipedia
- Wave-particle duality on Wikipedia
