# EM wave

1. Apr 6, 2005

### 7bear

In other medium except vacuum, there is different speed in different EM wave, right?
i.e. speed of radio wave is faster than speed of gamma wave, right?

2. Apr 6, 2005

### Integral

Staff Emeritus
Pretty bare bones question.

The bare bones answer is, No.

3. Apr 6, 2005

### ZapperZ

Staff Emeritus

In a dispersive medium, each frequency will have different speeds. This is because the index of refraction is frequency dependent, typically given as

$$n^2 = \frac{c^2k^2}{\omega^2}$$

where k is the wavenumber and $$\omega$$is the angular frequency of the light. Now since the speed of light in a medium is defined as

$$n = \frac{c}{v}$$

this means that each frequency of light travels with different speeds in the medium.

Zz.

4. Apr 6, 2005

### hemmul

However at some frequency range there exists also anomalous dispersion (if during the normal dispersion the refraction index is decreasing with the increase of the frequency - in the anomal case it is increasing)
So Integral's answer "No" is also applicable ;)

5. Apr 6, 2005

### ZapperZ

Staff Emeritus
Oy vey.

If we are going to have to consider ALL possible exceptions to the confines of what I think the question is asking, all our answers will be 20 pages long. I can also talk about materials with negative index of refraction, 1 and 2D materials with no well-defined quasiparticles that produce nowhere near the Drude peaks expected in conventional optical conductivity, etc.. etc.

Zz.

6. Apr 6, 2005

### hemmul

Sure, but that won't produce any new answers (besides "Yes" and "No") to the original 7bear's question ;)
i just implicitly meant that there is no general "Yes" or "No" answer to the question as is.

7. Apr 6, 2005

### hemmul

[offtop]
do you mean biisotropic media?
[/offtop]

8. Apr 6, 2005

### ZapperZ

Staff Emeritus
Yes there is: DEPENDS [and I don't mean the garment you wear for bladder control problems either!]. Depends is a superposition of "yes" and "no" answers since it can contain both simultaneously. (see, I can be a smart ass too).

And no, negative index of refraction, and left-handed materials are not all "biisotropic media".

Zz.

9. Apr 6, 2005

### jdavel

7bear really asked two questions. I think an accurate translation of into standard American English would be:

#1 The speed of an EM wave through a material medium is frequency dependent, right?

#2 The speed of radio waves through a material medium is less than the speed of gamma waves.

And I'm pretty sure that gamma waves pass through most materials almost as though they were a vacuum; radio waves certainly don't. So Integral's answer to #2 would be incorrect as well.

Last edited: Apr 6, 2005
10. Apr 8, 2005

### Mariko

gamma rays do not reach to the ground of our earth(thank you atmosphere and water particles) otherwise wed all be crispy critters thus it is not the gamma ray but of the atmosphere which blocks this wavelength of the electromagnetic spectrum as far as speed all electromagnetic waves move at the same speed period

11. Apr 8, 2005

### jdavel

Mariko,

"all electromagnetic waves move at the same speed period"

Really? Ever seen a rainbow?

12. Apr 8, 2005

### marlon

First of all : what is a 'speed period' ?

Secondly : NO, EM-waves do not have the same velocity in a certain medium. The velocity depends on the index of refraction n.

Thirdly : PHOTONS always travel at the speed of light.

regards
marlon

Last edited: Apr 8, 2005
13. Apr 8, 2005

### Integral

Staff Emeritus
My only response to this is OPPS!
I am not reading the question the same now, were I able to take it back I would, but it has been quoted way to many times. When I read it now I the answer I get is, Yes.

14. Apr 8, 2005

### Mariko

Yes I have and but what does a rainbow have to do with the speed at which a elecctromagnetic wave moves?Interesting i would like to know...Also
I ment that if that medium was space electro magnetic waves would move at the same speed which is the speed of light..And i dont know why mt text book here says this and yet you all want to tell me that its wrong?

15. Apr 8, 2005

### marlon

One single answer : (CHROMATIC) DISPERSION

marlon

16. Apr 8, 2005

### jdavel

Mariko,

"And i dont know why mt text book here says this and yet you all want to tell me that its wrong?"

No one here is saying that light speed through empty space depends on frequency. If they did, they'd be wrong.

But the original post in this thread was asking about light speed through "other medium except vacuum". And the speed of EM waves through material media depends on frequency.

This effect in rain drops is what creates a rainbow.

17. Apr 8, 2005

### Mariko

Yup ok thanks i was not reading the post right and I apologize for that thanks for clearing that up I think ill stick to the question asking side and let you pros do the helping heh