7

Optical refraction is related to the change in direction of a light ray when the refractive index changes. Excluding Earth's atmospheres and instruments, I think that refraction has little/no impact in astronomy. The only cases that comes to my mind where we can (probably) have some important refraction is in eclipsing star binaries or near edge on ...


6

Thermal radiation $\neq$ blackbody radiation. Thermal radiation is radiation that comes from a system where an equilibrium has been reached, where the various energy states are occupied according to the Boltzmann distribution and the particle velocity distributions are Maxwellian at some given temperature. That does not necessarily imply that the radiation ...


4

Temperature ($T_{eff}$) can be quite tricky to determine accurately as it interrelates to a number of other fundamental measurements. Firstly, remember that the spectrum we observe from stars are pin-point, they give us the entire overall result and not a specific location or part of the star. We need to dissect the various parts to arrive at the fundamental ...


3

Assume you have a spherical blackbody. The solar flux at the radius of the Earth is given to a good approximation by $L/4\pi d^2$, where $d = 1$ au. This is $f=1367.5$ W/m$^2$ (though note the distance between the Earth and the Sun has an average of 1 au). If it is a blackbody sphere it absorbs all radiation incident upon it. Assuming this is just the ...


2

By observing a source of light or radio waves that pass through the edge of a planet's atmosphere, the bending and the delay (both due to the lower speed of light in matter compared to vacuum) can be used to analyze that atmosphere. The four answers to the question When did planetary scientists realize Venus' surface pressure was almost 100x that on Earth? ...


2

Here's a paper I found which talks about the index of refraction of dark matter (different from gravitational lensing) and how a signal might attenuate. The paper is entitled, "Dark Matter Constraints from a Cosmic Index of Refraction", and here is the abstract: The dark-matter candidates of particle physics invariably possess electromagnetic ...


2

The question is compromised by saying that you allow arbitrarily perfect measurements. If we have a bolometer that can measure the amount of flux from a star, at a distance that is known to arbitrary accuracy, with arbitrarily good spatial resolution, then what we do is measure the bolometric luminosity from a 1 m$^2$ area at the centre of the stellar disk. ...


2

Your misunderstanding is in the second paragraph. If $n<n_c$ then there are insufficient collisions to excite the molecule into the upper level. Thus in equilibrium there are very few excited molecules and since the luminosity is proportional to the number of molecules in the excited level then little radiation is seen. The critical density is defined as ...


2

It is easiest to understand if you fix the incident angle and explain why the emergent intensity is largest at highly oblique angles. Since the light comes from the outside, it only penetrates so far, and this in turn causes the atmosphere to act like a source of scattered light. But the source of scattered light is brightest near the top of the atmosphere,...


2

The Gunn-Peterson trough is a feature in the spectrum of a (background) source near the Lyman $\alpha$ line, caused by a high density of intervening, neutral hydrogen (HI). The background source may be a quasar, but could also be e.g. a gamma-ray burst or just a regular galaxy with a strong Ly$\alpha$ line. Absorption vs. scattering The cross section of ...


1

We take a unit area dA of sun,, which is emitting light directly to our line of sight You are, I think, imagining the light as if it was directed out from a point, with directions of photons outward from the center. What actually happens is that the light is emitted from every point on the surface and what's more every point on the surface can send light ...


1

It is quite easy. In fact you do not need a bolometer. You just need to perform Intensity measurements in several parts of the spectrum, and then fit these to a teorethical black body spectrum. Three uses to be enough if it does not happen that you are measuring on a spike or valley in the spectrum caused by an emission or absortion line. The black body ...


Only top voted, non community-wiki answers of a minimum length are eligible