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bio website sjbyrnes.com
location Massachusetts
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visits member for 3 years, 1 month
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Jul
18
comment Total internal reflection and waveguides
Could you clarify, either (1) You want to learn how to derive the Fresnel equations, or (2) You want to know how you can calculate the angle-dependent phase shift starting from the Fresnel equations, or (3) Both.
Jul
14
comment How can molecule of a few angstroms absorb visible light of a few hundred nanometers?
If you want to understand the cross-section or linewidth of an atomic absorption line, then it is not a mistake to compare the size of the molecule to the wavelength of the photon.
Jul
14
comment How can molecule of a few angstroms absorb visible light of a few hundred nanometers?
I disagree. You can learn many interesting things by learning the classical theory of electrically-small antennas (e.g. en.wikipedia.org/wiki/Chu%E2%80%93Harrington_limit ), and applying those principles to atoms interacting with light.
Jul
14
comment How can molecule of a few angstroms absorb visible light of a few hundred nanometers?
I made an animation illustrating how an electron in a superposition state moves back and forth, just like you say in the first paragraph. See en.wikipedia.org/w/…
Jun
18
comment What could magnetic monopoles do that electrically charged particles can't?
There are two good questions here, incoherently mixed together. The first is: "What applications would a real magnetic monopole have?" The second is: "What applications do the magnetic monopole quasiparticles in spin ice have?" But these two questions are unrelated, see en.wikipedia.org/wiki/…
Jun
18
comment What could magnetic monopoles do that electrically charged particles can't?
Your first paragraph is describing the magnetic monopole analogue of a capacitor.
Jun
18
comment What could magnetic monopoles do that electrically charged particles can't?
The Particle Data Group article is about real magnetic monopoles (elementary particles); the ScienceDaily article is about magnetic monopole quasiparticles. They are unrelated, and it is misleading for you to suggest that they are the same thing. See en.wikipedia.org/wiki/…
Jun
18
comment What could magnetic monopoles do that electrically charged particles can't?
@WetSavannaAnimalakaRodVance -- You are misunderstanding Duality Transformations. Magnetic monopoles would behave differently than electric charges because the world is full of electrically-charged protons and electrons, which the magnetic monopoles would inevitably be interacting with. (Among other things.)
Jun
4
comment Why is the phase velocity used in the definition of the refractive index?
No. You cannot define index of refraction using group velocity. "Index of refraction" has a specific meaning in physics. It is a meaning that everybody learns and uses -- 100% of people, not 99%. Likewise, "group index" has a specific (different) definition. Let me ask you: "Why does the word 'velocity' always refer to the time-derivative of position, and never refers to the mass of Jupiter?" The answer is, because it's the way language works. Words have definitions. Otherwise communication would be impossible!
Jun
4
comment Why is the phase velocity used in the definition of the refractive index?
This is a strange question. If you want to talk about "c / group velocity", you call it "group index". If you want to talk about "c / phase velocity", you call it "index of refraction". It's just terminology! Those terms are as good as any. I think what you're really wondering is: "Why are there zillions of formulas that involve index of refraction, and very few formulas that involve group index?"
May
28
comment total noise power of a resistor (all frequencies)
@endolith -- Yes, I just said it was shorted because I wanted my question to be very concrete and specific. If you have a transmission line, it has a series of modes (standing waves), and in thermal equilibrium each mode has kT of energy (or less at high frequency). These modes exchange energy with a resistor: They give energy via joule heating, and get energy via johnson noise. This quantity 1.893E-12W/K2 is related to how fast the energy is exchanging. But, depending on what exactly you're calculating, you may need to take into account impedance matching etc.
May
13
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
Yes, heating a system increases its entropy largely because the velocity of each molecule has a greater range of possible values. There are other effects too: At higher temperatures, there is more uncertainty in how fast each molecule is rotating, and how much it is stretching or contorting...
May
12
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
The process creates heat. Wherever the heat goes, that's where the entropy increases. If the salad dressing is thermally insulated, the heat stays there, increasing the temperature and (thus) entropy. That's what user26866 is imagining. In the opposite extreme, the salad dressing might have negligible heat capacity compared to the surroundings, in which case all the heat spreads into the surroundings, so the entropy increase would occur in the surroundings. That's what Art Brown is imagining.
May
9
comment Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
you mean exothermic
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
You can calculate the group delay (aka "phase time") easily, e.g. using the classical transfer matrix method. And when you calculate the group delay, you'll find that sometimes it happens to be less than the speed of light over the thickness. So that is already a "theory that explains it", if you ask me.
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
When they say "no one has any explanation", they mean "no one has any intuitive explanation". Even more accurate: "no one has any explanation that I personally find to be sufficiently intuitive". There is no scientific mystery here: The math and physics of light propagation through multilayer coatings is extremely straightforward and well understood. They are stating a subjective judgment related to pedagogy. (A judgment which I would disagree with.)
Apr
27
comment Why do tunneling photons outrace their non tunneling counterparts in vacuum?
@user45342 -- Yes, I agree with all that.
Apr
25
comment Why use lasers for intense, localized heat instead of some other light source?
@BenRW -- You're right, I should have said "on the surface of the sun with a mirror overhead", for the maximum theoretically-possible intensity. I was wrong by a factor of two. However, if we're talking about laser cutters, you can't usually heat an object by illuminating it from all directions at once. If it's the surface of an object, you can only access it from one hemisphere obviously. And usually you illuminate it from a much smaller angular range than that. (A high-numerical-aperture lens would be impractical in a laser cutter for many reasons!)
Apr
25
comment Why use lasers for intense, localized heat instead of some other light source?
@BlackbodyBlacklight -- Yes, various flashtubes and discharge and arc lights have much higher intensity than focused sunlight (although still not nearly as much as a powerful laser). These have plenty of applications. But lasers are much more diverse. For example, for thermal emission, you get higher intensity by increasing temperature, but that also tends to decrease the wavelength towards UV. But lasers can also give you high-intensity infrared light, if that's what you want. And lasers can be solid, liquid, gas, low-voltage, high-voltage, ...
Apr
22
comment Dielectric constant of water
At low frequencies you can bet that the conductivity is independent of frequency. But that means that the imaginary part of permittivity is inversely proportional to frequency, i.e. goes to infinity as the frequency approaches zero. en.wikipedia.org/wiki/Mathematical_descriptions_of_opacity