Light tunnel question

[GuyWhoAsksQuestions]

I'm not a scientist and not sure if this is the correct location to post this, so please bear with me.

Just these random thoughts come into my head, so I have to ask.

 

I was reading about the inverse square law as it applies to light, and I have two hypothetical questions.

 

1. Assume there is a light source at one end of a long tube one million miles long, from which light could only escape through the opposite end.

 

Would the intensity of the light be equal along the entire distance of the inside of the tube?

 

2. Now assume you place a mirror over the end.

 

Would the intensity of light within the tube be ever-increasing?

 

 

Thanks

 

 

[Klaynos]

That would depend on the interaction of the light with the walls. If they are 100% reflective (impossible) and it was a perfect vacuum (impossible) then the intensity would be pretty consistent along the length.

 

I think you would find investigating how optical fibre works to be a very interesting activity.

 

 

Your second question, if we put the light source in the middle of our perfect tube with a mirror on each end then you would get an every increasing intensity inside. This is not physical though due to the limitations of reality.

[swansont]

To add to what Klaynos said, in very rough terms this is how an optical build-up cavity works. If you pop a gain medium in there, it's (again, omitting some details) how a laser works. The light that "leaks" out the end is the output of the laser.

[Klaynos]

I remember when I was younger drawing the design for a square with 45 degree mirrors at the corners, you'd input at the corner through one of the mirrors building up the intensity inside.

[fiveworlds]

Would the intensity of light within the tube be ever-increasing?

 

Assuming the particulate nature of light energy will be lost by the light over time. We know for instance that light will lose momentum. Therefore I would assume that eventually it will reach a critical maximum intensity.

Edited October 22, 2015 by fiveworlds

[swansont]

Assuming the particulate nature of light energy will be lost by the light over time. We know for instance that light will lose momentum. Therefore I would assume that eventually it will reach a critical maximum intensity.

 

Only if it interacts with something. An individual photon does not lose momentum on its own. Where would it lose it to?

[fiveworlds]

Only if it interacts with something. An individual photon does not lose momentum on its own. Where would it lose it to?

 

It interacts with the surface it is reflecting off. Every time it is reflected it loses a tiny amount of momentum in the collision. Which would lead to the red shifting of the light and then below the visible spectrum.

Edited October 22, 2015 by fiveworlds

[swansont]

Only if it interacts with something. An individual photon does not lose momentum on its own. Where would it lose it to?

 

It interacts with the surface it is reflecting off. Every time it is reflected it loses a tiny amount of momentum in the collision. Which would lead to the red shifting of the light and then below the visible spectrum.

 

If the mirror can move that will happen. But if it doesn't, where does the momentum go?

[fiveworlds]

If the mirror can move that will happen. But if it doesn't, where does the momentum go?

 

Same as when you hit a wall. The momentum goes to the particles in the wall which then convert it to other energy forms. Sound and thermal energy for example. I know from cooking that if you beat mixtures too long they heat up.

 

You can also cut a mirror with light :-)

Edited October 23, 2015 by fiveworlds

[BearOfNH]

So if you set up a square of 45° mirrors and used a http://www.laserpointerpro.com/1000mw-highpower-kaleidoscopic-blueviolet-laser-pointer-p-349.html (blue-violet laser), would you expect to see all the colors of the rainbow as the bouncing light gradually loses energy?

 

Come to think of it, that might be an interesting teaching tool at say the high-school level.

Edited November 19, 2015 by BearOfNH

[swansont]

So if you set up a square of 45° mirrors and used a http://www.laserpointerpro.com/1000mw-highpower-kaleidoscopic-blueviolet-laser-pointer-p-349.html (blue-violet laser), would you expect to see all the colors of the rainbow as the bouncing light gradually loses energy?

 

Come to think of it, that might be an interesting teaching tool at say the high-school level.

 

No. The photons would simply leave the system via absorption or transmission. Any effect of the sort fiveworlds described, if it actually happened, would be minuscule over the short time the light would survive. But if that actually happened, then you couldn't make high-finesse resonant cavities work — the frequency would shift and if it shifted by half of the free spectral range the light would have been lost —and we know they work. The loss rates are dictated by the reflectivity of the mirrors.

[John Cuthber]

 

Same as when you hit a wall. The momentum goes to the particles in the wall which then convert it to other energy forms.

No.

You can't convert momentum into energy.

(if you want proof, try looking at the units)

It has been specified that the walls are perfect reflectors. That requires them to have infinite mass (it's OK, we already said it was impossible)

So there's no momentum exchange on reflection.

[greeninter]

I think the farther away from the light source, the weaker the light will be.
For your second question, i think the intensity of light within the tube will be ever-increasing if you place a mirror over the end.

Just based on my life experience,