Physicist: This bothers a lot of people. When you’re learning physics, there are several things that you learn in the first couple years. Among them are:
1) The speed of light is an absolute.
2) Light slows down when it passes through a medium (like water, glass, air, …).
The first statement is the backbone of all of modern physics (relativity), and the second helps explain things like diffraction and rainbows. But clearly these statements contradict each other.
Here’s the idea: a medium, whatever it is, is made up of molecules. When a photon (light particle) hits a molecule it is sometimes absorbed. Its energy is turned into raised electron-energy-levels, or vibrations and flexing, or movement. In short order (very short order) the photon is spit out of the other side, none the worse for wear.
When a photon hits a molecule it's sometimes absorbed and re-emitted. That process takes a little bit of time that we interpret as a "slowing".
In between molecules light still travels at light speed. It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted. On the scale we’re used too, this happens so much and so fast that you don’t notice the starting-and-stopping. Instead you notice an average slowing of the light.
That is, if light always takes about 33% longer to travel through water than air (and it does) due to absorption and re-emission, you’d say “ah, light travels slower through water!”. The fact that that isn’t quite the case is rarely important.
Answer gravy: This isn’t part of the answer, but it’s interesting: The interaction between light and the medium it moves through is generally “clean”, in the sense that even if the light is in a complicated quantum state before entering the medium, it retains it. As a result, light continues to point in the same direction (which is good, in terms of seeing stuff), and even keeps its polarization.
What’s really fascinating, is that even more bizarre quantum states, like those involving being spread out over a large area, are also allowed to persist. If this were not the case it would be impossible to do the double slit experiment (which requires the photon to be in many locations) without a specially prepared vacuum chamber!
Why is it that the interactions between photons and the propagation medium are so clean?
“”In between molecules light still travels at light speed. It’s just that, with all those molecules around, it’s always darting ahead, getting absorbed, pausing for a moment, then being re-emitted.”"
so for a moment the velocity of light become zero???
It would be more accurate to say that, for a moment, the light is destroyed.
janne: The ‘clean’ interaction he described here is a special case for transparent materials. In general the emitted photon can have a different wavelength and direction from the incident one – this is called scattering. The type of scattering that occurs depends on how the incident photon’s energy compares with the electron energy levels in the molecule.
Forgive me if I’m just confusing concepts here, but how is refraction explained in this context? If one “side” of the light hits a new material and starts being absorbed/re-emitted a small period of time before the other “side” impacts, how does that change the angle?
Light does not speed up or slow down. The velocity of light is a constant for the medium through which it travels.
If speed of light changes in different medium then the kinetic energy(1/2mv^2) possessed by light should also change and it should be liberated in some way like energy or something else but I have never seen something like this happening?
Sure, but the point of the post was that the speed of light doesn’t change. The energy changes form, and the light starts and stops, but when it’s moving it’s moving at the usual speed.
Why is the speed of light finite? and why is it at the current speed? why not slower? or faster?
Q: Why is the speed of light finite?
As for why it is what it is? Who knows. You can make an appeal to the anthropic principle, but that almost certainly doesn’t explain the values of all constants.
light being absolute as in the speed of light , and i think but not sure as with falling in our atmasphere we have terminal velocity in space does mater have its own terminal velocity as light traveling through space has an maxium speed light speed but space is made up of matter and considering the big bang we are in a expanding universe so on the outskirts of our universe light and matter would travel faster ?
As far as we can tell, the universe doesn’t have “outskirts”. Every point is more or less the same as any other point.
As for light, it also behave the same way regardless of where it is: always moving at light speed.
The problem with this explanation is that it predicts that the time it takes light to traverse the glass should be directly proportional to the thickness of the glass. In fact, this is not the case–the speed of light through a particular type of glass is always a constant.
Why is it proportional to the thickness of the glass?
It is true that the time it takes to travel through glass is proportional to the thickness. But that implies that the velocity is constant, which is just the case!
Could space be a medium, being made up of dark matter? And it has a terminal velocity, the same way that matter can only “move air molecules out of the way” at a certain rate here on earth while being pulled by its gravitational pull, light can only do the same with dark matter, thus giving it what could be considered the terminal velocity of light through space?
Weirdly enough, the properties of spacetime (as we understand them) are incompatible with the properties of a material. The fact that the speed of light is the maximum speed isn’t due to some kind of property of space, in the way the speed of sound is dictated by the properties of air, it’s due to the fact that no matter how you move, light is always moving at the same speed. There’s an old post here that goes into more detail.