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location Massachusetts
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visits member for 2 years, 10 months
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Dec
1
comment Lasers and Collimation
I wrote "it's impossible to put 100% of [a laser's optical] energy into a perfect gaussian beam". This is obviously true because lasers have finite lateral size while gaussian beams do not. Are you really arguing that this statement is not true? Your comment above ("...not part of the beam") sounds like "100% of the energy goes into the gaussian beam, if you ignore the energy that does not go into the gaussian beam". Are you really arguing something so silly? If not, I don't understand you. The paper you linked to references a ~99.3%-efficient coupling, which they rounded to 100%.
Dec
1
comment Lasers and Collimation
@LukeBurgess - As you go away from the center of a gaussian beam, the intensity reduces exponentially, but never reaches zero in a finite distance. Therefore a finite-sized source or cavity CANNOT be 100% mode-matched to a gaussian beam. Maybe you can find a source that puts 99.99% of the energy into a perfect gaussian beam, but not 100%.
Dec
1
comment Lasers and Collimation
@LukeBurgess -- You have a funny definition of "collimated" if a wave can be "collimated at a single z-position". The whole point of collimating a beam is that it is supposed to stay collimated as it travels, until it hits the next lens. A beam of finite size can never be perfectly collimated. You can prove it by thinking about photons, or by working through Maxwell's equations, or by thinking about Huygens' principle, or by thinking about diffraction, or by thinking about fourier transforms and plane-wave decompositions. (Actually, these all amount to the same thing.)
Nov
30
awarded  Guru
Nov
27
awarded  Mortarboard
Nov
27
revised Definition of mean free time in the Drude model
deleted 89 characters in body
Nov
27
awarded  Good Answer
Nov
27
comment Photons from stars--how do they fill in such large angular distances?
@ArmanSchwarz - OK, I put the equation
Nov
27
revised Photons from stars--how do they fill in such large angular distances?
add equation
Nov
27
answered Definition of mean free time in the Drude model
Nov
26
awarded  Nice Answer
Nov
26
comment Photons from stars--how do they fill in such large angular distances?
This answer is sort of missing the point. It is correct to calculate a starlight photon flux, multiply it by the photon absorption cross-section of rhodopsin, and thereby calculate rhodopsin's photon absorption rate (as if the photons were bullets, randomly distributed in all directions). You are insinuating that there's something wrong with this procedure, but there's not. It gives the right answer! (I mean, the right answer for calculating light absorption rate.)
Nov
26
answered Photons from stars--how do they fill in such large angular distances?
Nov
24
awarded  Good Answer
Nov
17
comment Photons vs Uncertainty Principle
It is not correct that a photon has a certain exact momentum. Photons have uncertainty in momentum, and uncertainty in frequency, and uncertainty in wavelength, and uncertainty in energy, as well as in position and time!
Nov
17
answered Pn junction voltage drop?
Nov
14
comment Is an entropic force an actual force that can be explained as a fundamental interaction?
If you take away electromagnetic force, and strong, and weak, and gravity, then what's left? I'll tell you what's left: "A trivial system void of any interactions between its constituents". And we both agree that such a system has no entropic forces. Therefore "You need at least one of the four fundamental forces." Right? A lot of people get the confused idea that the "four fundamental forces" is not a complete list of fundamental forces, and we should include other forces along with those four. Please don't encourage that misperception!
Nov
13
comment Is an entropic force an actual force that can be explained as a fundamental interaction?
The excluded volume effect exists because of electromagnetism (primarily). Without electromagnetism, objects would pass through each other. So I don't see how this is a counterexample to the claim "Every entropic force is the manifestation of one of the fundamental forces".
Nov
12
comment Holes in a P-type semiconductor under external force E
No. In (B) you "Ignore the current from all the electrons in the valence band." Yes, all the electrons in the valence band, I really mean it. Ignore the electrons at the valence band maximum, ignore the electrons at the valence band minimum, ignore the electrons at the valence band middle ...
Nov
12
revised Holes in a P-type semiconductor under external force E
clarify - current --> valence-band current