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Hot answers tagged light

11

It's probably impossible to prevent chromatic aberration, since there is no such thing as a perfectly achromatic lens. but you can certainly minimize it. As you have mentioned, for a given spectral range, the best way is to choose a lens that is said to be "achromatic" even though the correction is never perfect. But if you want to use a lens that is not ...

8

Red shift is used for measuring the distance to very distant stars (galaxies mostly, in fact). The secret is to use spectral lines. Specific elements when very hot emit light at very specific colours and you can spot the pattern of those colours when the whole pattern has been shifted towards the red and see how far it has shifted. For closer stars, there ...

7

First, the Sun will not end up as a supernova - only a star $>8$ times the mass of the Sun will end its life in that way. You also have the wrong idea about "trapped light" (photons bouncing around and gradually working their way to the surface). Photons are constantly emitted and absorbed again and don't travel very far (compared to the radius of the ...

6

Peebles and Fukugita's excellent "The cosmic energy inventory" estimates that about $10^{-4.3}$ of the total mass-energy is in the form of cosmic background radiation, and $10^{-5.7}$ in the form of radiation from stars. The estimate that the perturbation due to plasma and other interactions of the background radiation is around $10^{-8.5}$, so clearly most ...

5

In summary "no, why would it?" If the universe continues to expand, then the photon will experience ongoing cosmological redshift, so it's wavelength (as measured by an observer moving with the general "flow" of galaxies in that part of the cosmos, will get longer, its energy lower, and it will, accordingly become more difficult to detect. We can detect ...

5

Photons travel on null geodesics through space time. In the curved space-time near a non-spinning black hole, a distant observer could infer that the speed of light (travelling on a radial path) is given by $$v_{\rm light} = c \left(1 - \frac{r_s}{r}\right),$$ where $r_s$ is the Schwarzschild radius. As $r \rightarrow r_s$ then we see that $v_{\rm light} \... 5 Assume it is possible to travel faster than c We normally cannot use physics to answer questions about scenarios that break the laws of physics. But for this scenario, we can say that if the ship can travel backwards through time, then it can escape from the event horizon. Just about every FTL scheme can be used for backwards time travel, and as far as we ... 5 A fouth approach to add to @Uhoh's 3 others, is to make the lens of extra low dispersion glass. Dispersion is a measure of how refractive index varies with colour. However such glass is relatively difficult to make, in the quality required for lenses, and so rather expensive. Further information about dispersion can be found on this wikipedia page https:/... 5 Canon camera has another method to reduce or cancel chromatic abberation they call Diffractive Optics. The idea is to use a lens with a diffraction pattern in it that causes the chromatic aberration to bend the opposite way from normal lenses. So if blue is bending the most through the normal lens, then it will bend the least when going through the ... 4 What's more, stars are so small to our eyes that they look like "point sources," meaning that each resolution element in our eyes gathers light from a much larger region of the sky than a distant star can fill. If you were to look at the Sun from, say, Uranus, and you have good eyes, it would still hurt almost as much because you could still resolve the Sun ... 4 How do radio telescopes gather information about visible light? They don't; the picture you saw all over the news was a false color image, where frequencies are shifted from the radio spectrum to the visible spectrum (blue meaning shorter radio wavelengths and red meaning longer radio wavelengths) and/or the intensities mapped to different colors (instead ... 4 The picture is of the central region of M87, taken at a wavelength where the gas is "optically thin". The ring of bright light is pretty much exactly where it is expected to be for the synchrotron radiation emitted by the hot gas to have been gravitationally lensed by a black hole with the same mass as deduced previously from the motion of stars close to ... 4 "Yes, but no." If you could jump through space faster than light can travel, and you went 1 light year away to 'look back at earth', then you would observe the light emissions that began 1 year ago, and continue to observe them as they arrive. [And if you start moving towards or away from the emission source, then 'neat things' happen due to blue/red shift. ... 4 The spectrum of light emitted by a star depends mainly on its surface temperature, with smaller variations associated with composition, surface gravity and magnetic activity$^{1}$. The surface temperature of a star is determined by its mass and evolutionary phase and therefore by its age. Stars spend most of their lives on the core hydrogen burning main ... 4 it's an interesting question. It could ask about any violent transient effect that produces light and gravitational waves, though "popping out of existence" is the ultimate step function. electromagnetic (EM) radiation is slowed very slightly by the presence of interstellar medium (especially by free electrons) and it's wavelength dependent. The image ... 3 I believe this question was addressed in the paper , Abbot, et. al., Gravitational Waves and Gamma-Rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A , ApJ. Lett., 848:L13, 2017 October 16. "We use the observed time delay of ($+1.74 \pm 0.05$) s between GRB 170817A and GW170817 to: (i) constrain the difference between the speed of gravity ... 3 What is the intensity distribution of visible light over the solar disk? What is the distribution of the visible spectrum over the solar disk Hopefully I've answered all the different questions. The core of the sun is extremely hot (27 million degrees Fahrenheit) compared to the surface (10,000 degrees Fahrenheit) and the atmosphere (36,000 to 2 million ... 3 Light (in general relativity) follows paths in spacetime dictated by the curvature of spacetime. That curvature is a result of gravitational distortion of spacetime by energy (and this includes mass as a form of energy). So light follows curved paths through curved spacetime. You should not, however, think of light as feeling a gravitational force in a ... 3 The (late time) ISW is caused by the evolution of cosmic structures as photons of the cosmic microwave background traverse them on their way to our detectors. It may cause a redshift or blueshift with respect to the redshift predicted for a homogeneous expanding universe. A bit more detail: If a photon "falls" into a potential well, its frequency and energy ... 3 There are two effects causing this: The relevant quantity for determining whether or not a photon interacts with a particle is the ratio $$x \equiv \frac{2\pi r}{\lambda},$$ where$r$is a size of the scattering agent. Rayleigh scattering When$x\ll 1\$, we're in the Rayleigh regime where the wavelength is so long compared to the particle that the ...

2

Albedo is a measurement of reflected light. It varies between 0 and 1, with 0 being completely black (reflecting no light) and 1 being completely white (reflecting all light). The albedo of coal is about 0.04, which is actually very similar to the albedo of our moon. The Albedo of ice is about 0.7. Only planets and other small objects are described using ...

2

I came across an article in the NOAO 2018 October Newsletter which discussed recoating of the 4-meter Blanco primary in Chile. This contained before, after and theoretical reflectivity data. I contacted one of the authors of the article for the source of the theoretical data who sent me a scan of the pages from R. N. Wilson's "Reflecting Telescope Optics II",...

2

Sure it does, and it's beautiful! Here's a GIF from the YouTube video Earth from Himawari-8 satellite which I found in but any geosynchronous Earth observation satellite imagery will show this, such as the video Planet Earth in 4K found in this now-deleted answer. See also: this answer to Could a binary system of two planets with oceans reflect each other?...

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A glory is a rainbow-colored bullseye appearing directly opposite the Sun due to diffraction by water droplets of a certain size. From an airplane, you can sometimes see this effect surrounding the airplane's shadow. As the NASA article explains, the polar orbiting Terra satellite's scan pattern transformed the circular rings into elongated ovals.

2

You could do better than that. You could walk around on a younger earth. This Physics.SE Q&A addresses why. The short of the matter being that things are moving with respect to each other, and so have different inertial reference frames. Your ability to travel faster than light lets you exploit the differing reference frames to conduct a round trip ...

2

The sun is one "astronomic unit" away. The unit is defined that way. The closest other star is Proxima Centauri at a distance of 268,770 astronomic units away. The intensity of the light reduces at distance squared; if Proxima Centauri sent out just as much light as the sun we would see it as (268,770)2 times fainter. That is approximately 72 billion times ...

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Distance is important, as is surface brightness, but there's also one more thing that has not been let onto here. First, a recap. For one, the fact that while total brightness falls with distance, surface brightness does not is indeed that - an unassailable fact, as simply observing whether or not the surface brightness of a wall changes as you move closer ...

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@Astrosnapper's answer to Helioseismology of the Sun, what is actually measured? begins with: The two main methods to detect the solar p-mode oscillations, which have a period of about 5 minutes (so frequency of ~3 milliHertz) are line of sight Doppler shift and change in irradiance. 3 mHz is the peak of the power spectrum, but a 2D histogram of shows ...

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There are various ways of measuring distances in the universe which depend on the range in which we are measuring. Parallax method: Up to 100 - 1000 ly Exploiting period luminosity relation of Cephied variables: Up to 3 - 10 Mpc Tully Fisher relationship: Intergalactic distances Hubble parameter: Large scale structures in the universe Magnitude measurement ...

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