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How far can light travel on earth or in the atmosphere?

Or, if I want to be more specific, how far can one photon travel until it disintegrates?

I assume gravitational pull and other effects cause it to vanish after a while. Let's say a light beam or laser-beam that has many trillions of photons are shot some distance up from the atmosphere. So, the light rays makes a spotlight on the earth. (I.e. the light is shot in all directions.) Hence, makes a spotlight. If you were a flat earther, you would base your theories on this idea that the spotlight makes the illusion of a spherical earth.

Is there some math equations that knows approximately how far a packet of photons can travel in atmosphere?

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    $\begingroup$ Photons do not disintegrate. Some photons have traveled through space for billions of years! They do get scattered and/or absorbed by molecules in the atmosphere, however. $\endgroup$ – G. Smith Jul 16 at 4:40
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    $\begingroup$ This part is not clear to me: "Lets say a light beam or laser-beam that has many trillions of photons are shot some distance up from the atmosphere. So, the lightrays makes a spotlight on the earth." Is the source of this light beam on the ground or in the sky? Which direction is it aimed at? $\endgroup$ – PM 2Ring Jul 16 at 7:41
  • $\begingroup$ The center of the lightbeam is somewhere up in the sky, directed towards earth. $\endgroup$ – Natural Number Guy Jul 16 at 15:49
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It depends on your definition of light. Radio waves are a form of light, and some optically visual photons from the early universe are redshifted to radio wavelengths by the time they reach us. So how far can photons travel in the Earth's atmosphere? About half way around the world, surprisingly enough. In space, and in the atmosphere, photons usually travel in a straight line, but are sometimes steered off course by gravitational fields. The Earth's gravitational field is too slight to bend light rays to any significant degree, but they obviously have to be bent to reach the other side of the world. There is an ionised layer of the atmosphere between 50 and a hundred miles up called the Heaviside layer, which reflects radio waves and enables them to follow the curvature of the Earth, but so far as I know they can only get about halfway around before being too attenuated to be of further use.

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Photons are elementary particles in the SM.

When a photon interacts with an atom, three things can happen:

  1. elastic scattering, the photon keeps its energy and changes angle

  2. inelastic scattering, the photons keeps part of its energy and changes angle

  3. absorption, the photon gives all its energy to the atom

There are two cases:

  1. visible light

Now when in your case, the light you shoot interacts with the atmosphere, all three happen. Most of the photons will have Rayleigh scattering (elastic scattering), that is what gives the sky its blue color. Some photons will scatter inelastically, or be absorbed, thus heating the atmosphere.

  1. non-visible light

The ionosphere is what decides between low and high frequency EM waves. Low frequency radio waves can travel around the Earth, because the ionosphere reflects them back. Hich frequency EM waves will travel through the atmosphere. These can make it to infinite distances, if they do not interact with anything else.

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The fact the we can see the sunrise shows that visible light can travel very far through the atmosphere. Twice that distance is about the longest distance that light would be able to travel inside earth's atmosphere due to the radius of the earth. The fact that lunar eclipses are red shows that (red) light can travel that distance through earth's atmosphere.

Now, that fact that light is reddish when we see the sunrise also tells us that light can be scattered by the atmosphere (same reason why the sky is blue). Blue light is scattered more readily than red light, through a process called Rayleigh scattering.

Light can also be absorbed. The amount of absorption also depends on the wavelength. Visible light is not absorbed very much, but when you go to infrared or ultraviolet light then you find absorption bands in the atmosphere.

Photons do not "disintegrate," because they do not consist of other particles. They are absorbed by charged particles, in which case they completely disappear. They can be re-radiated however.

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