# Where do photons go?

Discussion in 'General Electronics Chat' started by cjdelphi, Jul 14, 2009.

1. ### cjdelphi Thread Starter New Member

Mar 26, 2009
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If we're seeing light from stars that are not only millions and millions of miles away from our own planet but the light we say maybe hundreds of years old then I'm presuming light can travel great distances

Now what happened if you took a beam of light and two very reflective surfaces, would you not (with a strong enough light source) be able to bounce the beam back and forth between the two mirrors or whatever for sometime to come after switching the light off?.

Mirror 1 >>>> Mirror2
Mirror 2 >>>> Mirror1.

back and forth?...... if not where's the light go?

2. ### RiJoRI Well-Known Member

Aug 15, 2007
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Sounds like you are describing part of a laser.

My guess is that because we do not have perfectly (100%) reflective surfaces, some of the photons are bounced out of the mirror-mirror path and wander out into the wide universe.

--Rich

3. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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Now keep in mind that light acts like a wave and also as particles (photons or light quanta), but never at the same time. While a wave needs a medium to travel through, particles don't. Originally, people thought that light was a wave, and it worked out great in mathematical calculations, but how would it travel through space if there is no medium? That's why people thought that there was "ether" in space, which was conjectured by the Greeks. Finally, they proved that light, radio waves, IR waves, UV waves, and visible light acted as waves and also as particles. I would recommend that everyone on AAC reads "The Story of Science" by Joy Hakim! It's a great book!

4. ### studiot AAC Fanatic!

Nov 9, 2007
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Yes that is indeed what happens.

Lucas' comment about a 'galaxy far far away along time ago' is also true the original souce may even have ceased to exist by the time it gets to us as the first mirror. Even then the light can be reflected on and continue until it is finally absorbed somewhere in

a galaxy far far away and a long time ahead.

5. ### Mark44 Well-Known Member

Nov 26, 2007
626
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A water wave needs a medium to move through, just as a sound wave through some material does, but a light wave doesn't need a medium in which to travel. At least that's my memory of what I learned in Engineering Physics a long while ago. Light is an electromagnetic phenomenon, with oscillating electrical and magnetic components (E field and B field, I believe they were called). The presence of these fields is what enables light to travel through a vacuum, unlike other types of waves.

Thought of as a particle, a photon doesn't really "bounce" off anything. When a photon strikes an object, the energy it is carrying is transferred to the struck object, which excites some outer-shell electron to a higher-energy state, thereby emitting another photon.

I think what I have said here is correct, but again, it's been a long while since I had those Physics classes, so I might be amiss on a detail or two.

6. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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The thing about a photon is that it's an isotropic source. You can detect a photon from ANY direction with equal sensitivity. This is as you'd expect from any point source of radiation....it attenuates with distance as the wave spreads out in 3 dimensions.

What can we say about a PARTICLE that's detected from any direction from its source? A particle can only travel in one direction!

Hmmm....this wave/corpsuckle duality gets interestinger by the second!

eric

Apr 5, 2008
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Nov 26, 2007
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9. ### ELECTRONERD AAC Fanatic!

May 26, 2009
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Indeed, light is pure massless energy. Although, it comes in the form of corpsucles (light quanta) and also as a wave; but never at the same time. When light hits metal, it knocks the electrons around. Electrons (believe it or not), also comes in the form of a wave and also as corpsucles. How does light know when to change from a wave to particles or vice verce? That is still yet unkown; scientists are still trying to figure that out.

10. ### Mass Member

Apr 9, 2009
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When you say a photon is massless, that simply means it is not affected by a gravitational field, right?

But gravity bends light (as in black holes), does it not? I'm a little confused.

Last edited: Jul 15, 2009
11. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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That's a good point!

Apr 20, 2004
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13. ### ELECTRONERD AAC Fanatic!

May 26, 2009
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Thanks beenthere,

I admit that I wasn't quite sure about that myself! Thanks for clarifying!

14. ### Mark44 Well-Known Member

Nov 26, 2007
626
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And although photons are considered to be massless, they have momentum according to the formula p = $\hbar$ k, where $\hbar$ is the reduced Planck constant, and k is the wave vector. See the wikipedia article on photons for more information.

One proposal for long-distance travel within and outside the solar system is to use very large "light sails" that would be take advantage of light's momentum by being pushed by the photons streaming from the sun.

15. ### ELECTRONERD AAC Fanatic!

May 26, 2009
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Yes, indeed. Because in space, photons take the form of particles. Very interesting!

16. ### studiot AAC Fanatic!

Nov 9, 2007
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Actually photons are not massless.

They have zero rest mass. But no one has ever seen a photon at rest, or ever will.

The rest (ha ha) of the photons in the universe, which are all mobile, posess some mass, by virtue of their velocity.

There are many simple terrestrial and extra terrestrial experiments which demonstrate this, eg the 'light pressure vane' which spins in a vacuum driven by differential light pressure.

17. ### ixisuprflyixi Active Member

Sep 16, 2007
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I didnt notice an answer to the OP's question so I conclude the answer to be; Yes you can bounce light between two mirrors even after the light source has been turned off as long as, like you said, the light source is "strong enough" to overcome the losses that the electromagnetic radiation would have from spreading out in all directions and the losses from the mirror surfaces not being 100% efficient at reflecting light. In order to lower the effect of the loss of light this experiment would likely be carried out over a shorter distance but with high speed and accuracy.

18. ### beenthere Retired Moderator

Apr 20, 2004
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If the mirrors are perfect and are perfectly alligned, then the photons will be reflected for an infinite length of time. Think of front surface mirrors with no dispersion due to glass.

19. ### rin-elt New Member

Jun 18, 2009
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as far as i can remember photons have a dual action!
but they don't simply bounce off surfaces,this is a question of energy possessed by the photons!
lignt is massless and not weightless so as it travels in a meduim it must feel the effect of the G-field!!

20. ### jpanhalt Expert

Jan 18, 2008
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Are you sure the radiometer to which you refer will operate in a complete vacuum? I have read that the rotation is due to differential heating and the effects of residual gas, not the push from photons per se.

John