# Speed of light and air resistance

1. Jun 18, 2008

### roxberry

Does the speed of light slow down due to air resistance? I often hear about how the speed of light is constant in a vacuum, and I'm assuming it has something to due with it being different in air due to air resistance. If so, why is this? Since light has no mass, shouldn't air not have an affect on its speed?

2. Jun 18, 2008

### gendou2

No. Air resistance is a macroscopic effect of molecules bouncing off one another.

In the air, light scatters off the air molecules, which takes time. Think of the air molecules as speed bumps.

A photon has zero rest mass, but it most certainly has momentum!
Heat is the motion of molecules, right?
Well, what does sunlight do to the air?
It heats it up. Three cheers for global warming!
This means that light entering Earth's atmosphere is absorbed by molecules in the air.

3. Jun 18, 2008

### roxberry

I'm not getting what you're saying here. It sounds like you're saying light takes longer to get from point A to point B because it has bounced off of molecules and hasn't gone in a straight line. If that's what you're saying, the photons themselves have still gone at a constant speed, no?

4. Jun 18, 2008

### Nick89

Yes. Afaik photons themselves cannot move in any speed different from c. In a medium however they interact with the medium in such a way that the light appears to slow down.
For example, light from the center of the sun can take about 5000 years (I believe, not 100% sure) to reach the surface of the sun. This does not mean that the sun is 5000 lightyears large! It means that if you would turn on a lightbulb in the sun (ignoring the fact that it would burn up ofcourse and assuming you can see it) it would take 5000 years for you to be able to see it outside the sun.

5. Jun 18, 2008

### roxberry

Okay, let me see if I have this straight:

The speed of light is constant. When one makes the claim that light is slower, in, say water, than in a vacuum, it's because it only seems slower because the light photons did not end up at point B as fast as it would have in a vacuum because it bounced around and thus did not travel in a straight line.

Can others confirm that this is correct?

6. Jun 18, 2008

### dst

Yes, it can change direction but not speed.

7. Jun 19, 2008

### gendou2

No. Photon scattering (even if the direction is unchanged) takes time.

8. Jun 19, 2008

### Nick89

Yes that is roughly what I tried to explain. What do you mean by 'no'?

9. Jun 19, 2008

### Quisquis

AFAIK the extra time is from the absorption and the re-emission of the photon though, not the travel time of the photon itself.

I think of it like this:

Imagine a car that can accelerate to 88 MPH instantly, and it can go from that to zero instantly as well.

If that car is on the freeway, it will always be traveling at 88 MPH. Consider that the speed of light in a vacuum.

If that same car had to take city streets it would have to stop at lights along the way (maybe take a detour or two as well) and therefore when it moved, it would always be moving at 88 MPH, but it would still take longer to get to it's destination than on the freeway because it had to interact with the environment around it. Consider that the speed of light through a medium.

Last edited: Jun 19, 2008
10. Jun 19, 2008

### Cthugha

No! This is as wrong as it gets.
You would notice drastic differences, if absorption occured.
There is also a good explanation of this in the forum FAQ here:
https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]

Notice, that the explanation in the FAQ is for solids, so it is not 100% the same here, but the reason, why absorption is not the cause of light slowing down, stays the same.

Last edited by a moderator: May 3, 2017
11. Jun 19, 2008

### Logics21

question :D
i don't want to open a new topic

how is it that they managed to stop a photon completely, my highschool phisics teacher mencioned it.
did light stop in frame or what?

12. Jun 20, 2008

### Nick89

Like I said they didn't stop a photon. (Btw did they really stop light or just slow it down very much?)

You cannot stop a photon, nor can you make it move at any speed different than c (so not slower and not faster). What you can do is construct a material with which light interacts in such a way that it appears to slow down. The analogy of QuisQuis is a good one, a few posts up.

EDIT
Maybe you mean they trapped a photon in a cavity? Then still, the photon is moving, it is bouncing around in the cavity with a speed of c.

13. Jun 20, 2008

### Cthugha

No, it is not that easy. They fire some probe pulse at some cold gas of sodium or a similar material, which is usually opaque for light with the wavelength of interest.
Now you use another laser beam to introduce EIT (electromagnetically induced transparency), which - roughly speaking - switches off the transition, which would absorb the probe beam, by destructive interference of the probability amplitudes for different "paths", in which the mentioned transition can occur. The gas becomes transparent for the probe beam, while the other laser is switched on.

Now you switch off the other laser, while the probe beam is in the middle of the gas. It becomes opaque again and there is no way out for the probe beam. The information of the probe beam (phase, amplitude) is now transferred to the spin state of the gas atoms. If you switch the pump laser back on, the material becomes transparent again and the probe beam comes out again.

Whether you should call nondestructive storage of this information stopped light or not, is a different topic, but this is - in easy terms - how it works.

14. Jun 20, 2008

### Nick89

But still there is no case of a stopped photon, right? All photons involved are still moving at c?

15. Jun 20, 2008

### ZapperZ

Staff Emeritus
It depends entirely on what you mean by "stopped".

Stopping light is a common occurrence. When you light hits an opaque object, it is stopped. However, as has been explained, the Lene Hau experiment where her group "stopped" light has an added significance. Here, not only is the energy stored in the gas, but also the phase coherence is also preserved, to be "replayed" later. So they held all the properties of that light for a length of time before it is sent out. So this is what we call stopping light.

Zz.

16. Jun 20, 2008

### Nick89

By stopped I mean the speed of the photon being 0 m/s. Or for that matter anything less than c m/s.

17. Jun 20, 2008

### Alfi

The explanation gives me an image that shortest route, and therefore shortest time, from a to b is in a vacuum.
Any interaction with energy can only lengthen the time from a to b. like the absorption, re-emission as you mention.
Do photons, bending on their way to us, but not interacting, at the edge of the Sun from it's gravitational influence, travel a longer distance but get here at the same time?

18. Jun 21, 2008

### Andy Resnick

The speed of light is given by c = c_0/n, where c_0 is the speed of light in a vacuum, and n is the refractive index. This is a continuum model- if you want to talk about individual scattering events, you can- but getting c_0/n out of that model is incredibly complicated and ultimately pointless. Better to ask about the molecular basis for refractive index.

Polychromatic light packets can be slowed down to arbitrarily slow speeds by making 'dn/d(wavelength)' very large; which it is under conditions known as 'anamolous dispersion'. Light can also be converted to spin; this has also been done to 'stop' light, store it, and release it at a later time.

I also read about a project involving a highly scattering powder which acted like a "roach motel"- light went in, and didn't come out. The phenomenon required lots of closed scattering loops to be present.

19. Jul 17, 2008

### Pumblechook

Light and other forms of EM radiation travel more slowly in mediums other than a vacuum. I think light is 50% slower in glass. But the difference between a vacuum and air is tiny. Similarly radio waves travel slower in coaxial cable linked to the dielectric constant of the dielectric (insulating material) within the cable.