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Tags light particles , photons

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Old 5th July 2009, 10:49 AM   #1
Southwind17
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Probably barking completely up the wrong tree here but I'm intrigued to learn the explanation, whatever it is:

Taking our main natural light source, the Sun, as an example, assuming photons are particles, and that it's photons emitted from the Sun that excite our retinas, thereby enabling us to observe the Sun, given that the Sun is observable from everywhere within an imaginary sphere of space billions of kilometers in diamater at the same time with the Sun at its centre, wouldn't that require almost infinitely more photons than the Sun comprises, or generates?

Moreover, assuming light (again, presumably photons) travels at a constant speed, the Sun, again for example, presumably would be observable from all of those places where its photons have reached since it came into being (using a powerful/sensitive enough measuring device, of course).

Finally, if photons are indeed particles, then what happens to all of those photons flying around my bedroom when I turn off the light at night? Perhaps it explains that constant accumulation of "dust" in our homes! I guess the answer to this might well answer the foregoing questions, if somebody cares to go for the easy option!
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Old 5th July 2009, 11:07 AM   #2
Gate2501
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Originally Posted by Southwind17 View Post
Finally, if photons are indeed particles, then what happens to all of those photons flying around my bedroom when I turn off the light at night? Perhaps it explains that constant accumulation of "dust" in our homes! I guess the answer to this might well answer the foregoing questions, if somebody cares to go for the easy option!
As I was saying in another thread, it is really helpful to think of things like gravity/light, as an ever expanding sphere, emitted from the source.

The particles of one emission make up one wave, or layer of the sphere, like a bubble. The light in your bedroom is constantly pumping out new expanding bubbles, when you turn it off, it stops, and the expanding bubbles of light that are present currently in your bedroom expand away, or get absorbed by surfaces(as heat), at the speed of light.

Right?

I'm a newb so someone else might give you a better more technical answer.

Last edited by Gate2501; 5th July 2009 at 11:09 AM.
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Old 5th July 2009, 12:01 PM   #3
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Originally Posted by Gate2501 View Post
As I was saying in another thread, it is really helpful to think of things like gravity/light, as an ever expanding sphere, emitted from the source.
This works fine for me, so far as it goes, but leaves me wondering why there isn't a photon at every place within that [light] sphere, given that the light source can be observed from every place within it.

Originally Posted by Gate2501 View Post
The particles of one emission make up one wave, or layer of the sphere, like a bubble.
Can you elaborate on what you mean by "one emission" and "one wave". Presumably you're alluding to the frequency? Which, if so, has me wondering, and might help: is the frequency the rate at which photons are being emitted, i.e. in pulses? Or am I confusing wave length?!

Originally Posted by Gate2501 View Post
The light in your bedroom is constantly pumping out new expanding bubbles, when you turn it off, it stops, and the expanding bubbles of light that are present currently in your bedroom expand away, or get absorbed by surfaces(as heat), at the speed of light.
So not dust in the morning then?

Thanks
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Old 5th July 2009, 12:18 PM   #4
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Originally Posted by Southwind17 View Post
This works fine for me, so far as it goes, but leaves me wondering why there isn't a photon at every place within that [light] sphere, given that the light source can be observed from every place within it.
But it can't. The intensity of light at a particular point can be thought of as the number of photons passing through a unit of area centered about that point.

Suppose the total number of photons emitted at an instant from the light source is constant. Let's call this constant number N. The surface area of a sphere at a distance R is , which means that the number of photons per unit area is

As you can see, this goes down with increasing R, so as you move further away, there are fewer photons per unit area. Therefore you need a larger detector to be able to see the light source. If your detector is too small, and you're far enough away from the light source, it's possible that you won't be able to see the source at all. This is why you need huge telescopes to see faint stars - you need to be able to intercept enough photons from that star to get a meaningful signal.

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Old 5th July 2009, 12:20 PM   #5
sol invictus
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Originally Posted by Southwind17 View Post
Taking our main natural light source, the Sun, as an example, assuming photons are particles, and that it's photons emitted from the Sun that excite our retinas, thereby enabling us to observe the Sun, given that the Sun is observable from everywhere within an imaginary sphere of space billions of kilometers in diamater at the same time with the Sun at its centre, wouldn't that require almost infinitely more photons than the Sun comprises, or generates?
No. Your eye (or a photodiode) detects light in discrete packets - photons. At any place and over some time interval there is some probability of detecting zero photons. Obviously "close" to the sun the probability is extremely low, but far enough away it becomes non-negligible.

Think of it as like a continuous explosion that's forever throwing huge numbers of tiny particles out. Far enough away, those particles aren't very dense and you or your detector might fall through the cracks for a whole (so to speak).

Quote:
Finally, if photons are indeed particles, then what happens to all of those photons flying around my bedroom when I turn off the light at night?
They get absorbed by the walls, furniture, etc. Other photons are constantly being emitted by them as well, but so long as your room isn't so hot it's glowing you won't be able to see those, although you might feel them coming from a radiator (the wavelength is too long for the human eye to detect).
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Old 5th July 2009, 10:12 PM   #6
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Originally Posted by theturtlemoves View Post
As you can see, this goes down with increasing R, so as you move further away, there are fewer photons per unit area. Therefore you need a larger detector to be able to see the light source. If your detector is too small, and you're far enough away from the light source, it's possible that you won't be able to see the source at all. This is why you need huge telescopes to see faint stars - you need to be able to intercept enough photons from that star to get a meaningful signal.
I understand this, but there are clearly some photons, nonetheless, at every observable place.

Originally Posted by sol invictus View Post
Your eye (or a photodiode) detects light in discrete packets - photons. At any place and over some time interval there is some probability of detecting zero photons. Obviously "close" to the sun the probability is extremely low, but far enough away it becomes non-negligible.
Not quite sure what you mean by "close", but every star that I look at in the clear night sky remains constantly visible. I suppose in stellar terms all such stars could be considered "close", but even so, for there to be many photons from each such star where I'm standing (and at every other place in each star's observable sphere) that's a hell of a lot of photons.

I suppose I'm simply underestimating the sheer intensity and hence number of photons being emitted at the source.
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Old 5th July 2009, 10:24 PM   #7
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Originally Posted by Southwind17 View Post
I suppose I'm simply underestimating the sheer intensity and hence number of photons being emitted at the source.
It's something around 1046 photons per second from the Sun. And a sphere with radius of 8 light years has a surface area of about 1038 square inches. Still a lot of photons per square inch.

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Old 5th July 2009, 10:54 PM   #8
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Originally Posted by RecoveringYuppy View Post
It's something around 1046 photons per second from the Sun. And a sphere with radius of 8 light years has a surface area of about 1038 square inches. Still a lot of photons per square inch.
I was also thinking of the old candle on a clear night adage too, which possibly makes my point even better than the Sun example. Do you have scientific data for that one also?
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Old 5th July 2009, 10:56 PM   #9
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Originally Posted by Southwind17 View Post
I understand this, but there are clearly some photons, nonetheless, at every observable place.

Not quite sure what you mean by "close", but every star that I look at in the clear night sky remains constantly visible. I suppose in stellar terms all such stars could be considered "close", but even so, for there to be many photons from each such star where I'm standing (and at every other place in each star's observable sphere) that's a hell of a lot of photons.

I suppose I'm simply underestimating the sheer intensity and hence number of photons being emitted at the source.
There really aren't photons at every observable place from the sun, it's just that your eye is sufficiently large to collect huge numbers of them from the distance of Earth. Imagine instead that your eye was the size of an atomic nucleus. It would be entirely possible for photons to miss your eye, so that the sun flickered on and off continuously.

Perhaps another way of looking at this is to compare it to a puff of air hitting your eye (a glaucoma test, for instance.) You know that that puff is made up of a huge number of atoms (well, molecules mostly, but still) and that the atoms collide with your eye and the interaction creates the sensation of something blowing in your eye. Even though there must be spaces between those atoms you still feel it. The same is true of light striking your eye; even if there are gaps in the light, your eye will still pick up the signal. (The 'gaps' in this case would be remarkably tiny in either example.)

The stars that you look at in the night sky are those that are either very close or very bright. In the first case, enough photons reach your eyes to register as a star because they haven't spread apart too far and in the second there are so many photons being emitted that they can spread out more before falling below the threshold at which you would no longer be able to see them. Even in the darkest sky, a person with good vision can only see a few thousand stars distinctly. With a larger 'eye' such as a small telescope or binoculars, that number jumps dramatically. When you get to a big telescope and extremely sensitive recording equipment, you can see remarkably distant objects. In the famous Hubble deep field images, there are entire galaxies that were only detected by the telescope catching a handful of photons over several (11?) days because their light had spread so thin.
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Old 5th July 2009, 11:15 PM   #10
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Thank you jasonp for that - it certainly seems to confirm my supposition. BTW - by "every place" I really wasn't meaning at the atomic or even molecular level - just "eye level", but the numbers are still bind boggling.
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Old 5th July 2009, 11:58 PM   #11
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Get far enough away from a light source, be it a candle or a star, and you won't be able to see it, for the reasons that Sol outlined above. This is one reason why we have so much difficulty seeing ancient stars & galaxies billions of light years away. Note that in order to take photographs of those objects, it may require exposure times that are literally weeks or months long to gather up enough photons to resolve an image with our eye.
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Old 6th July 2009, 04:32 AM   #12
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Originally Posted by MattusMaximus View Post
Get far enough away from a light source, be it a candle or a star, and you won't be able to see it, for the reasons that Sol outlined above.
Sure - my point is, though, that it's a darned long way, in any direction (theoretically), meaning that a small amount of candle wax (in the case of candle light) is somehow converted, presumably, into photons which, when emitted, can then kind of "fill" a massive space!
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Old 6th July 2009, 05:10 AM   #13
sol invictus
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Originally Posted by Southwind17 View Post
Sure - my point is, though, that it's a darned long way, in any direction (theoretically), meaning that a small amount of candle wax (in the case of candle light) is somehow converted, presumably, into photons which, when emitted, can then kind of "fill" a massive space!
A single optical frequency photon carries an energy of a bit less than 10^-18 Joules. A watt is a Joule per second. So if for simplicity we take a 1 watt light source that emits only visible light, it would be sending out roughly 10^18 (a billion billion) photons per second.

That's a lot.
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Old 6th July 2009, 05:18 AM   #14
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Originally Posted by sol invictus View Post
A single optical frequency photon carries an energy of a bit less than 10^-18 Joules. A watt is a Joule per second. So if for simplicity we take a 1 watt light source that emits only visible light, it would be sending out roughly 10^18 (a billion billion) photons per second.

That's a lot.
It sure is, and I'm starting to see the light now!
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