# What is the end of a light ray?

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#### prabhavathi

##### Guest
Hi

Does a light ray ever die?
Sun light travels even after Pluto. But what happens at the end?

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#### MeteorWayne

##### Guest
There is no end, unless all the photons are absorbed by particles, billions of light years away.

A

##### Guest
Light follows the Inverse Square Law. When energy radiates away from a source, its intensity is inversely proportional to the square of the distance from the source.

As MeteorWayne observes, it's going to keep going on an on appearing dimmer and dimmer inversely proportional to the square of the distance.

Stand six feet from a wall. Step half the distance to it. You're now 3ft from the wall. Take another step half the distance to the wall. You're now 1.5ft from the wall. Now step half the distance to it again and you're 0.75ft from the wall. How long until you reach the wall? Forever. You never will. Now you know how light, heat, other radiation feels.

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#### prabhavathi

##### Guest
I can now understand light ray.

I am sorry for being foolish on the following question.

Same sun light reaches earth and pluto. What makes earth to keep warm?

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#### andrew_t1000

##### Guest
In a vacuum do photon's just keep going?

If we were to look at a single photon of say 650 nm red light, if it never hit or interacted with anything, would it keep going?
And still have it's 650 nm wavelength?

Which brings to mind another question.

I've read that our eyes can "detect" a single photon.

What would that "look" like?

A brief flash?

Just wondering.

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#### MeteorWayne

##### Guest
Yes, in a true vacuum, a photon keeps going forever. In reality, there are molecules and dust in between galaxies, so eventually the photon will be absorbed and reemitted at a different frequency.

I don't believe that the human eye can detect a single photon. While it is absorbed, the signal is too small for the eye-brain system to detect.

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##### Guest
prabhavathi":1zwzttlq said:
I can now understand light ray.

I am sorry for being foolish on the following question.

Same sun light reaches earth and pluto. What makes earth to keep warm?

Prabhavathi,

There is nothing "foolish" about your questions. They are the basis for all _good_ questions about how our planet (and universe) function around us.

First we need to understand the layers of the atmosphere.

The layer of the atmosphere that we live in is the Troposphere. A rule of thumb is that it's from 0-10Km (varies from pressure and how close to the equator we are. It's just a handy number - at the equator it's about 7Km and 17Km at the poles). Then the Tropopause, then the Stratosphere ( to 51Km), then the Stratopause, then the Mesosphere (to 85Km) [which is the layer where most meteors burn up], then the Thermosphere (to ~350ish Km) [this is where you'll find the ISS], then the Exobase (notice, no Thermopause. The Exobase is the beginning of "space"), then the Exosphere.

Our troposphere is primarily heated by the transfer of energy from the surface of the earth, so the warmest part is the part closest to the surface (in general). Thermal radiation from our sun is absorbed into the surface of the earth and reflected back up into our Troposphere. Water vapor in the troposphere absorbs, reflects, refracts, and otherwise transfers heat back down towards the earth and into the water vapor surrounding it.

It's very very cold in the Tropopause because there is little water vapor and it's furthest from the surface of the earth. This is, incidentally, why deserts are so cold at night. They are very dry, so they don't have the water vapor to store the heat. During the day, the sands are good reflectors and reflect the heat right back up to the tropopause. You're standing at the focal point of the reflector, so it feels hot as the heat is bouncing back around you.

The Stratosphere is almost dry, so it behaves like a desert at night. It maintains a constant but chilly temperature.

Entire books can be (and have been) written on the subject of how the layers of our atmosphere behave. This is a very simplified overview, but hopefully gives you enough information to research further if you're still interested.

Why is Pluto so cold? Well, for one, it doesn't have an atmosphere to hold heat in as we detailed above. For the second, we noted that radiation (like light and heat) lose power following the Inverse Square Law. It's not "light" that heats the earth or Pluto - the photons aren't interacting that way. It's radiant heat, a different radiation. Regardless, all radiation still follows the Inverse Square Law. The intensity of that radiation (heat) is inversely proportional to the square of the distance from the source. If we're 8 light-minutes from the sun, at 64 light minutes from the sun the intensity of all the radiation will be the square-root of what we experience here. Pluto is around 330 light minutes from the sun (that changes with orbit, of course), about 41 times further away.

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#### MeteorWayne

##### Guest
prabhavathi":2pxp9clo said:
I can now understand light ray.

I am sorry for being foolish on the following question.

Same sun light reaches earth and pluto. What makes earth to keep warm?

Prabhavathi,

There is nothing "foolish" about your questions. They are the basis for all _good_ questions about how our planet (and universe) function around us.

Pluto is around 330 light minutes from the sun (that changes with orbit, of course), about 41 times further away.

And therefore gets ~ 1/1680 of the energy that earth receives.

A

##### Guest
Thank you for continuing that. I got a work call and just kinda rushed through the end. Probably lucky, the post would have been 4x the length...

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#### MeteorWayne

##### Guest
NP, that's why we're a team here :lol:

A

##### Guest
Gotta love Tag Team Physics.

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#### zorski

##### Guest
MeteorWayne":3439iggw said:
prabhavathi":3439iggw said:
I can now understand light ray.

I am sorry for being foolish on the following question.

Same sun light reaches earth and pluto. What makes earth to keep warm?

Prabhavathi,

There is nothing "foolish" about your questions. They are the basis for all _good_ questions about how our planet (and universe) function around us.

Pluto is around 330 light minutes from the sun (that changes with orbit, of course), about 41 times further away.

And therefore gets ~ 1/1680 of the energy that earth receives.

Hi, would you mind showing the math on how you came up with 1/1680? I've been trying to figure it out but I don't think I understand how you used the inverse square rule. Thank you

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#### MeteorWayne

##### Guest
41 squared is 1681.

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#### Mee_n_Mac

##### Guest
andrew_t1000":3tsmyb3n said:
In a vacuum do photon's just keep going?
If we were to look at a single photon of say 650 nm red light, if it never hit or interacted with anything, would it keep going? And still have it's 650 nm wavelength?

Just to add to what's been said (if a bit too late) ...

Your 650 nm photon would not be 650 nm after travelling a few billion lightyears of distance. Because the universe is expanding, the wavelength will be stretched as well. That's why far away galaxies display cosmological redshift.

http://en.wikipedia.org/wiki/Redshift

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#### andrew_t1000

##### Guest
Ok, good point!

What actually happens in laser pointers that use a crystal to down-shift infra-red light to green light?

I love threads like this where we all learn something!

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#### Mee_n_Mac

##### Guest
andrew_t1000":1l64793z said:
Ok, good point!

What actually happens in laser pointers that use a crystal to down-shift infra-red light to green light?

I love threads like this where we all learn something!

I knew something of the basics for green laser pointers but I had to look up the details. They're based on Diode Pumped Solid State Frequency Doubled (DPSSFD) laser technology (now there's a mouthful !). A high power IR laser diode at 808 nm pumps a tiny block of Nd:YVO4 generating light at 1,064 nm, which in turn feeds a intracavity frequency doubler crystal to produce the green beam at 532 nm. So the IR laser's output get's downconverted into a longer wavelength light which then is upconverted into green light. Details on this latter process can be found here ...

I'd have to read it about 5 more times before I'd could comment on it reasonably. :lol:

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