length of a beam in the sky?

Say you shine your high powered light (15MCP, POB, Maxabeam) in mid air.

Like so... Not my light, just good example.


You notice the beam just stops at some point with a smoothed rounded edge.

So whats going on here?

Id assume the tapered tip of the beam would really stretch out faintly for km/mi? Same thing with high powered lasers.

Nice beamshot Flash! Is that your 6000K Mega Illuminator? That is just what my POB's beam looks like.

I know my POB's beam is visible out to 1 mile away, so I would guess it appears over 1 mile high. However, I am also curious about how far it really goes, the light must travel miles up into the sky (theoretically it travels on forever).

I could be mistaken, but I think that rounded tip of the beam is just an illusion since it is going so far that you can't see where it ends. I notice when I walk away from the beam and view it from 30 yards away it looks much longer than when I am close to the light, as I can see how far it is really going up.

When I shine my green laser up into the sky its beam looks much longer than the POB's beam and appears to go much farther up. I know the laser's beam stays concentrated miles up into the sky and hits even the highest clouds, although the dot is too small to be visible at that extreme distance.

I would be interested to know how far away a spotlight's beam is actually visible from, like how many miles up could I see my POB's beam shining directly at me? Probably several miles!

nah that is the orig Harbour freight, just can't remember which thread it came from.

I tried taken beam shots of my VT's whole beam & almost did my neck looking straight up from 30m away. :laughing:

I need to get a camera that can capture the whole beam from around 30 m away. Which is hard since all cheap dig cams are like 35mm + lens.

So I lie down on grass & try take as steady shot as possible :kiss:

I'd say that the beam "ends" at the limit of the moisture rich layer of air.
The fairly dryer upper layer of air will not cause any scattering and the beam "ends"
Sure, the light carries on until it reaches a thick enough cloud layer or the end of the universe but it's scattering is visible only for a few hundred feet. :thinking:
Greet's .

Here is my theory I think that it is just an optical illusion.



The angle between the "end" of the beam and guy #1 eyes is so small that he actually can't see it. Guy #2 is very far from the light source and looks from side on it. He should be able to see the disappearing end of the beam.

hahah great illustration Jirik_cz :thumbsup:

:wave:

the other day I was watching a police choppper fly around shining it's light down on the ground, and they straight ahead of the chopper. The beam was easily hitting the ground (you know, kind of what it was made for) but when it was shining it forwar, it just tapered off, but didn't seem to go as far as to the ground was shining. It's got to be some optical illusion going on.

The beam really does end suddenly at a certain distance. There is an explanation in Feynman's "Lectures on Physics" but it's been years since I read that book and I don't have a copy any more, so I don't remember specifics. It is fairly understandable and very cool though. I should check if my local library has it and I might look that topic up.

I expect Feynman has a better explanation but there's mine:

It can all be explained mathematically (rough order of magnitude calculation)

x = distance from source
I= perceived light intensity
~ means "proportional to"

There are a few factors to consider:

1) Absorption by cloud/atmoshpere of incident light
I ~ 1/x

2) inverse square law on incident light
I ~ 1/(x^2)

3) inverse square law on reflected light
I ~ 1/(x^2)

4) Absorption by cloud/atmoshpere of reflected light
I ~ 1/x


This gives a 1/(x^6) relation. A function that falls off very suddenly. (OK, some of that maybe redundant but you get the idea).
Now consider this optical illusion: The beam expands in diameter as the distance increases but perspective makes it look roughly the same size. This disguises the perspective i.e. distances at the top of the beam are in fact much greater than they appear. This makes the beam intensity look like it's falling off much more rapidly.

These two factors make the beam appear to suddenly terminate.

EDIT:

The beam doesn't actually terminate in my model though. It just drops off very rapidly. Perhaps Feynman meant a beam in fog? Where no rays of light can escape the atmosphere without encountering a particle of water and being scattered randomly?

great replies.

amazing it just suddenly stops. thats pretty cool.

scottaw said:

the other day I was watching a police choppper fly around shining it's light down on the ground, and they straight ahead of the chopper.

Click to expand...

Did they catch you ? ...... Sorry, couldn't resist

monkeyboy said:

The beam expands in diameter as the distance increases but perspective makes it look roughly the same size. This disguises the perspective i.e. distances at the top of the beam are in fact much greater than they appear. This makes the beam intensity look like it's falling off much more rapidly.

Click to expand...

The length of the beam will depend on the power of the light. Really powerful lights push their lumens out of the front with much more force, so they go further, whereas the lumens from weaker lights are more feeble and they don't go as far. If you point them up in the air, you can sometimes see weak lumens falling back to earth (you need to look through a polarising filter to view this effect). However you won't see that happening with really strong lumens – they just keep on going until they are right out of sight.

DM51 said:

The length of the beam will depend on the power of the light. Really powerful lights push their lumens out of the front with much more force, so they go further, whereas the lumens from weaker lights are more feeble and they don't go as far. If you point them up in the air, you can sometimes see weak lumens falling back to earth (you need to look through a polarising filter to view this effect). However you won't see that happening with really strong lumens – they just keep on going until they are right out of sight.

Click to expand...

Light does not work that way. In this situation you can think of light in terms of quanta of energy or "photons". A brighter light achieves greater intensity by putting out a greater rate of photons. The energy carried by each photon depends only on wavelength not intensity.

A photon will travel indefinitely until it interacts with something. The trajectory of a photon can be affected by gravity but the earth's gravitational field is far to weak to have any noticible effect.

the length of the beam will depend on the power of the light.

Click to expand...

i.e "THE MAXABEAM"?

Do you maxabeam owners notice the beam alot longer then say a 35w HID coscto or Ti Mega?

great point Monkey & thanks again :thumbsup:

you can sometimes see weak lumens falling back to earth

Click to expand...

like a ball thrown up in he air & falls back down once it runs outta puff? LOL

Ah, someone else still draws stick people. Now I do not feel so artisticly challenged.

The "weak lumens" effect is actually quite pronounced on lights with older Luxeon emitters, many of which are now showing signs of deterioration due to Chip Weevils.

righto, Mr Leg puller :laughing:

Doh!

The beam from the Luxor hotel in Vegas seems to reach out to infinity. Must have something to do with the powerful arrays being tightly focused.

scottaw said:

the other day I was watching a police choppper fly around shining it's light down on the ground, and they straight ahead of the chopper. The beam was easily hitting the ground (you know, kind of what it was made for) but when it was shining it forwar, it just tapered off, but didn't seem to go as far as to the ground was shining. It's got to be some optical illusion going on.

Click to expand...

[OT]this should interest you in terms of what that heli might be using
https://www.candlepowerforums.com/threads/167229[/OT]