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GR8677 #3
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Alternate Solutions |
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Comments |
rehab
2009-10-27 18:13:50 | the answer must be (B )otherwise groub velocity will exceed the speed of light from this realtion vp=c^2/vg and that ,of course, impossible!!rn |  | smokwzbroiplytowej
2008-10-22 19:18:33 | Not _exactly_ related to this problem, but Wikipedia has a nice applet showing the difference between group and phase velocity:
http://en.wikipedia.org/wiki/Group_velocity
| | blue_down_quark
2008-08-02 09:10:29 | I think the answer is incorrect. The question doesn't ask 'what is light's velocity ?' it instead asks 'what is light's PHASE VELOCITY? '. Phase velocity can actually be larger than c . The equation for phase velocity is   =  . I think the correct answer is B . for more information see : http://en.wikipedia.org/wiki/Phase_velocity
wangjj0120
2008-08-20 20:59:02 |
but when is the equation v_pv_g = c^2 valid?
To integrate this equation, we realize that
only when the light propagate in the air and wave guide (or plasma) have the dispersion relation which corresponds to this equation.
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zaijings
2009-04-03 08:22:28 |
I am also confused when the problem asks about the phase velocity.
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segfault
2009-08-22 21:06:49 |
I think when dealing with a signal of a single frequency (e.g., blue light), it only makes sense to talk about the phase velocity--you can't construct a "group" or wavepacket out of a single frequency signal.
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jchys
2010-02-02 20:38:17 |
If you look at the section carefully, it says that
"any particle that has MASS (according to special relativity) phase velocity of matter waves always exceeds c"
photons obviously has no mass.
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| | erc
2005-11-04 09:11:07 | I don\'t know anything about this, so maybe about to make a fool of myself...
But, to be consistent, I think the and  should each lose the power?
Thanks.
yosun
2005-11-04 15:10:14 |
erc: thanks for the typo note. The extra squared sign on the c's and v's have been removed. (you can also check it by dimensional analysis. the units for permeability and permittivity are given on the table of info included with the exam)
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|  | erc
2005-11-04 09:09:06 |
rehab
2009-10-27 18:30:09 |
the answer must be (B )otherwise groub velocity will exceed the speed of light from this realtion vp=c^2/vg and that ,of cource, impossible!!
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