# Does the Speed of Light Change depending on different Light Frequencies?

1. Jul 31, 2005

### PhysicsEnthusiast

We all know that light speed in a vacuum is constant and travels 9.8 X 1018 meters in a year. Now since light travels in waves instead of a straight line, if the wave was then uncurved, it would cover more distance than when it was in its original state. Of course different lights have different frequencies so a wave with a higer frequency must cover more ground than waves of lower frequencies like red.

But if a radiowave was put next to a lightwave and both where pointed to the same destination, based on the Einstein's constant rule, both will hit the wall at the same time. My questions is, is the higher frequency wave faster than the lower frequency wave since it had to cover more distance to hit the same destination at the same time? Does this mean that the speed of light is different at each frequency?

2. Jul 31, 2005

### Spin_Network

No, A low energy radio wave, inside a Vacuum, will propergate at a constant Velocity, equal to that of the High Frequency Lightwave.

Inside a Vacuum Chamber they are indistinguishable, a Low energy RW gains velocity from the Vacuum Pressure.

3. Jul 31, 2005

### ZapperZ

Staff Emeritus
No, it doesn't. If it does, than Einstein would have put in his postulates the PARTICULAR frequency in which he claims to have a constant speed.

The varying frequencies dependent is typically more of the consequence of the MEDIUM, not on light. This is where the frequency will make a difference. The fact that we have not detect ANY difference in speed of light in vacuum[1,2] for any frequency is an argument against the idea that light travels in a "medium" when it moves through vacuum.

Zz.

[1] "Severe Limits on Variations of the Speed of Light with Frequency", B. Schaefer, PRL v.82, p.4964 (1999). See summary at http://www.aip.org/enews/physnews/1999/split/pnu432-2.htm [Broken].

[2] M. Fullekrug, PRL v.93, p.043901 (2004).

Last edited by a moderator: May 2, 2017
4. Jul 31, 2005

### PhysicsEnthusiast

I read his article. I was wondering what (Delta c/c), (Mgamma), and (Eqg) stand for.

For example:

- for the frequency that is (Delta c/c), what numbers do I use to replace those inside the brackets?

- for the quantum gravity that is (Eqg), what is the given number?

- for the photon's mass (Mgamma), since photon's have zero mass, would I replace the "Mgamma" witha zero?

Thanks.

5. Jul 31, 2005

### HallsofIvy

First, your argument about "straightening" the wave and looking at the length of that doesn't make much sense. Light consists of electro-magnetic waves, not waves in any physical substance that can be "straightened".

As ZapperZ said, while the speed of light in vacuum is constant, in some medium other than vacuum, it does depend upon frequency. That's why different colors of light bend different angles going through a prizm.

6. Jul 31, 2005

### Gokul43201

Staff Emeritus
I think the OP imagines that light travels along a sinusoidal path - so the greater the frequency, the greater the path length, and hence, the greater the "actual" speed.

Also, straightening out, must refer to stretching out the sine curve to a straight line.

7. Aug 4, 2005

### DaTario

You perhaps will profit from considering a wave of temperature in a large plane metal plate. The possibly sinusoidal shape of the propagating temperature profile have nothing to do with the distance the peak of the wave travel during a given time interval. Electric field is not until now (to the extent of my knowledge, of course) related to metric. But I believe it may be involved in it, but this is only a speculation.

Note: temperature propagates in a plane metal plate according to a different equation, not the wave equation. But even so, I think, this fictitious example may clearify some points in this discussion.

Last edited: Aug 4, 2005
8. Aug 5, 2005

### Danger

As Halls and Gokul have alluded to, it appears to be a misinterpretation of the wave nature of EM. The light does not follow a sinusoidal path; it is one (2 actually, since the electric and magnetic fields are separate). The only way to flatten it is to decrease the frequency to zero, which will automatically increase the wavelength to infinity in order to maintain the mandated speed of 'c'. In other words, shut the lights off.

9. Aug 5, 2005

### bomba923

Hmm---but what if not in a vacuum?

I'm learning Physics C (from Barron's Review, I admit!), and I am not yet at the EM chapters. However:

Would higher frequency EM waves travel at speeds closer to $c$?--->
I mean,
*Let's say you have a tank of still water, and you shine some light through it. Water will interfere/"slow down" the light passing through it, and its speed will be less than $c$. Now, you shine another EM wave through it, this time one with HIGHER frequency than the original light you shined through the tank.
*Will the higher frequency light be "Less" affected by water's resistance?? (More energy to the beam, more mass--if that concept of relativistic mass applies??) WIll the Higher frequency light experience/feel "less" resistance to the water than the original light?---and will thus beam faster through the tank? Anyone??

Last edited: Aug 5, 2005
10. Aug 5, 2005

### Pengwuino

No, they will still travel at teh same speed. Light has no mass so there is no inertia to be affected by the water.

11. Aug 5, 2005

### Danger

That's sort of a yes-and-no situation. By definition, whatever speed the light is travelling at is 'c'. While there will be no inertial interactions, the refractive index of any medium will affect the travel characteristics of light passing through it. For instance, X-rays will take a lot less time getting through a piece of oak than red light will.

12. Aug 5, 2005

### Pengwuino

Hmm why does that happen. I thought they would just go slower then 300,000km/s but would both go at the same speed..

Wait i take that back. I still dont get it. How can they slow... wait... ok im sleepie, ill read something in the morning.

Last edited: Aug 5, 2005
13. Aug 5, 2005

### Danger

The speed of the individual photons is the same, but some are able to take a more direct route than others so the forward propogation speed varies.

14. Aug 5, 2005

### bomba923

But suppose we have a block of wood. If both rays enter normal to the block, and neither is angularly deflected via refraction,
*then both rays will pass through the block at the same speed regardless of frequency?

But doesn't wood/water/etc... "slow down" the speed of the light to less than $c$ ? Wouldn't a higher frequency beam observably feel/experience less resistance from the medium (w/more E per photon)?

Last edited: Aug 5, 2005
15. Aug 6, 2005

### Danger

If you're not familiar with the game "Pachinko", this might not make sense, but I'm going to use it as an analogy anyhow. In that game, a bunch of steel balls about 5mm in diameter are simultaneously dropped down a vertical playfield with a bunch of pegs, traps, etc.. along the way. (Like a vertical low-tech pinball without flippers.) Although each ball is accelerating downward at 1g, they all experience different interactions on the way. If one were lucky enough to miss everything, it would drop from top to bottom at a constant 1g. Every time one is deflected, however, adds to its total travel time. It doesn't slow down to any appreciable degree, but the detours make it stay in play a lot longer. They all eventually hit the bottom, but at widely varying times.
In the case of EM radiation, the higher the frequency the fewer 'pegs' it hits on the way through.

16. Jul 18, 2008

### Pumblechook

Does the Speed of Light Change depending on different Light Frequencies?

Yes, NASA and other space agencies have found very slight differencies (which surprised them).

Space seems to a cause a 'group delay' problem and means that parts of signal (different sidebands) arrive before others causing distortion.