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bio website daniel.farrell.name
location Tokyo, Japan
age
visits member for 3 years, 9 months
seen Sep 29 at 19:17

I know how to use the grey square -- do you?


Jul
9
comment Anti-reflective coating effect on total internal reflection?
At the opposite side of the TIR the electric field of the light wave is non-zero; is has a spatially decaying solution. This is know as an evanescent wave. If you have another surface very close to the TIR surface then some of the evanescence waves amplitude will enter the other medium. This is frustrated TIR.
Jul
7
comment Most useful maths for theoretical and mathematical physics
I'm a lurker there, but have seen advice type questions. Will have to check the small print.
Jul
7
comment Most useful maths for theoretical and mathematical physics
Since this has been closed here you might want to ask the same question at the academia stack.
Jul
7
comment What is the difference between emission wavelength and lasing wavelength?
Thanks for clarifying, this is my understanding too.
Jun
25
comment Entropy of photons
Interesting question. I can't write a definite answer without research, so I'll add a comment. I don't think you can define the entropy of a single photon (or a single particle for that matter) because entropy is a property of an ensemble. In the case of photons entropy is related to the electronic properties of the material emitting the light (temperature of the electron gas) because photon themselves cannot interact to reach equilibrium. I'm sure there are more rigorous definitions and I hope others will comment.
May
30
comment A photon travels in space for 10 billion years. What are the odds it will arrive here without interacting with a atom on the way?
The probability is $\exp\left(-\alpha d\right)$ where $\alpha$ is the absorption coefficient along the photons path and $d$ is the distance travelled.
May
28
comment state occupation rate $n_{i}=\frac{1}{e^{\beta (\varepsilon _{i}-\mu )}+{[1/-1/0]}}$ & density matrix $\rho _{m}=\frac{e^{-\frac{E_{m}}{kT}}}{Z(T)}$
Why do you call them occupation rates, they don't have units of $s^{-1}$?
May
28
comment Reflection vs indices of refraction
The refractive index of the liquid is 1.47. Search for Fresnel reflection.
May
19
comment What will I see in this scenario? Will this be faster than the speed of light?
If I film myself smashing a glass, does playing the movie backwards break the second law of thermodynamics?
May
13
comment Optical Spectra and the Diffraction Grating
To add a bit more context. The position (wavelength value) of the spectral lines is known, so this allows you to determine a relationship between the angle if your diffraction grating and the diffracted wavelength. Once calibrated you can measure an unknown sample an be sure that the wavelength values you determine are correct (once converted from the grating angle).
May
13
comment Optical Spectra and the Diffraction Grating
To add a bit more context. The position (wavelength value) of the spectral lines is known, so this allows you to determine a relationship between the angle if your diffraction grating and the diffracted wavelength. Once calibrated you can measure an unknown sample an be sure that the wavelength values you determine are correct (once converted from the grating angle).
May
9
comment How can we know that a black hole exists?
So nothing below the Chandrasekhar limit yet?
May
6
comment When the matter is heated does the photon absorbed by electrons of atoms or by atoms themself?
I think the question is more about the mechanism by which energy is transferred to matter from photons. The answer is good. I'd like to emphasise that this is an electromagnetic process, photons (at IR and visible frequencies) only couple to electron cloud around atoms not the nucleus. So in a nutshell photons are absorbed by electrons. The actual details are material dependent as described in this answer: there are rotational, vibrational and electronic degrees of freedom.
May
5
comment Do reversible processes exist in Nature?
Some quantum systems can reversible flip between two states, for example Benzene rings. But this is far from the statistical definition of entropy that you are interested. I'm not aware of any physical system or thermodynamic engine which can be made completely reversible (i.e. operate at the Carnot limit), it's probably impossible. Carnot efficiency give the maximum efficiency of a heat energy and therefore the minimum amount if entropy that needs to be generated.
May
1
comment How is the Earth heated by a Full Moon?
Thanks! Try another way if you can think of one, hopefully we get the same value.
Apr
29
comment Solving coupled nonlinear ODEs, controlling numerical instability through numerical viscosity
This question is perfect for scicomp.stackexchange.com if you don't get a reply here.
Apr
25
comment In thermodynamic systems why must the free energy of the system be minimized?
I'll take a go, this is incomplete but might serve to start some discussion. The free energy is special because it define an energy at which you can add/remove particles without a change in entropy. The actual value of free energy is dependent on the physical properties of the system, but once the free energy is minimised you have maximised entropy, so the system is in quasi equilibrium (just not yet in chemical equilibrium).
Apr
17
comment Free electron gas in two dimensions
Oh yes, very good!
Apr
17
comment Free electron gas in two dimensions
I don't know if there is an intuitive reason for this...
Apr
17
comment Temperature in a Voltaic Cell
Do you mean a chemical battery?