GR | # Login | Register
  GR8677 #72
GREPhysics.NET Official Solution    Alternate Solutions
Verbatim question for GR8677 #72
Wave Phenomena}Group Velocity

Recall the definition of index of refraction n in terms of the speed of light in vacuum c and the velocity of the light in medium v, n=c/v. Since the velocity of light in any medium is v\leq c, the condition n \geq 1 usually holds. However, even if rock salt has n > 1, the wave does not exceed the speed of light. The group velocity can travel faster than the speed of light, and apparently it is the group velocity at work in the equation n=c/v.

One can also arrive at this conclusion via MOE (Method of Elimination):

(A) Relativity is a pretty general theory that supposedly applies everywhere. (Newtonian mechanics can be achieved using the proper approximation technique.) X-out this choice.

(B) An x-ray is a specific frequency in the electromagnetic spectrum. Nothing forbids an electromagnetic wave from transmitting signals, and thus this choice is out.

(C) Imaginary mass? If confused, save for last comparison.

(D) Historical precedence shouldn't change the correctness of a theory (at least not in the ideal world ETS lives in)... X-this out.

(E) One recalls that there's a difference between group and phase velocities. Could this difference allow one of them to exceed the speed of light? Probably. In either case, this is a much better choice than the other remaining candidate, choice (C). So, choose this.

See below for user comments and alternate solutions! See below for user comments and alternate solutions!
Alternate Solutions
There are no Alternate Solutions for this problem. Be the first to post one!
2009-12-18 12:55:18
Refractive index= C/V
group velocity is always less than C, so refractive index is less than one. But phase velocity can be more than C, hence refractive index can be greater than one. so the result E makes sence. For better explanation, please see the link below.

2009-12-18 13:20:28
Please ignore the above explanation.
Refractive index= C/V, group velocity is always less than C, so refractive index is greater than one. But phase velocity can be more than C, hence refractive index can be less than one. so the result E makes sence. For better explanation, please see the link below.
2008-10-17 11:37:38
I have to say I just laughed out loud at the "imaginary mass" choice :)
2010-11-03 12:29:03
yea me too, now I'm wondering if there is any deeper meaning to this other than a mere pun.
2013-10-18 20:08:13
Actually this answer does kind of make sense. If n<1, then since v=\frac{c}{n}, it seems like the velocity of the photons is greater than c. Recall that for a massive object, E=\gamma mc^2. But \gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}. So if v>c, then \gamma is imaginary! And since E must be real, that implies that m is imaginary. Of course photons have zero mass so this is all nonsense, but this answer is actually the most plausible of all the other wrong ones.

Objects that travel faster than c are called tachyons. Laugh all you want, but a lot of string theorists believe that tachyons do exist, and that they have imaginary mass.
2006-11-16 12:10:49
No! The group velocity v_g cannot be greater than c, and it is not v_g which is "at work in the equation n=c/v". Concerning the latter, if v=v_g, then we have


which is called the "group index", and which is NOT the commonly known refractive index n. Instead, n is defined with respect to the phase velocity v_p=\frac{\omega}{k}.
2007-10-28 13:21:58
To clear things up, the index of refraction n=c/v_phase. rnrnn<1 does not violate the theory of relativity, which says that no information-carrying signal can propagate faster than c, because it is v_group that describes the velocity of the transmitted information.
2007-10-28 13:22:43
That's just supposed to be n<1 in the second line.
2008-10-28 15:31:02
speed of light in a material is less than c; so, the group velocity in the medium can be greater than the the speed of light (for that medium).
Jeremy P
2008-11-04 10:53:01
nitin is right.rnrnV_{group} carries information and so cannot travel faster than crnrnV_{phase} carries no information and is greater than crnrnuse n=\frac{c}{V_{p}} to arrive at n<1
2011-10-27 01:43:48
I do agree with the guys who believe this is the phase velocity which can be faster than the speed of light. We also got familiar with this fact in electromagnetic and optics..
2006-10-29 22:19:21
Shouldn't it be n<1 for the 4th line?
2011-08-07 08:10:14
yes it must have been n<1
2014-09-18 03:53:50
correction: no it is true in the answer.....n>1 so vc why? this was the question.
Typo Alert!

Post A Comment!
Click here to register.
This comment is best classified as a: (mouseover)
Mouseover the respective type above for an explanation of each type.

Bare Basic LaTeX Rosetta Stone

LaTeX syntax supported through dollar sign wrappers $, ex., $\alpha^2_0$ produces .
type this... to get...
$\langle my \rangle$
$\left( abacadabra \right)_{me}$
The Sidebar Chatbox...
Scroll to see it, or resize your browser to ignore it...