Why won't mirrors reflect light perfectly?

[taylaron]

how exactly do lasers work?
i know there are many kinds of lasers, but why can't you shine a light in a sphere with the inside as a mirror, then leave a hole and put a long tube on the hole, so the rays going sideways won't escape
i hear that the light just won't stay inside the sphere. it won't "build up"
why won't mirrors really reflect light. not just to some degree.

 

[Danger]

Hi, Taylaron;
I don't have much time here, so I'm just going to hit on it briefly. In a laser, photons hit electrons, which knocks the orbitals up. Since the electron doesn't want to be there, it drops back down to ground state. In order to do so, it has to release the same amount of energy that kicked it up in the first place. That is accomplished by the emission of a photon identical to the one that started all the trouble in the first place. When all of this is happening inside a compartment that is fully mirrored on one end and partially mirrored on the other, you end up with a whole bunch of identical photons bouncing around. Eventually, they line up perpendicular to the mirrors. They keep bouncing back and forth between them, stimulating even more photonic production, until they're strong enough to bust out through the partially reflective one. The thing that makes them a laser, rather than just a monochromatic flashlight, is that all of the EM waves are perfectly in synch with each other (coherent).
As for the mirror question, remember that reflection is simply a special form of refraction. You don't see the original light coming back. In much the same way as the laser works, the incoming light screws up electrons, which then re-emit the same frequency 180 degrees opposite to the photons that hit them.
I'm sorry that this isn't a proper explanation, but I really have to get to bed now.

 

[juming]

the incoming light screws up electrons

... well said!

 

[berkeman]

And to reinforce Danger's explanation:

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

 

[taylaron]

elaboration

so,
technically, the laser is doing a series of "burst fires" because each burst has to charge up to reach the specific sync which they then leave the partially reflective mirror.
when you start the photonic production, you shoot photons into a chamber with 2 mirrors (1 on each end) as to get them all going the right direction/in sync, but when you start this, your not constantly adding more and more photons are you? I am confused when you said

In a laser, photons hit electrons, which knocks the orbitals up. Since the electron doesn't want to be there, it drops back down to ground state. In order to do so, it has to release the same amount of energy that kicked it up in the first place. That is accomplished by the emission of a photon identical to the one that started all the trouble in the first place.

does this mean your starting out with a specific amount of photons which are hitting electrons, where are the photons hitting the electrons on a charged plate/mirror? i didnt know photons had electrons orbiting itself.
do the emissions of photons come from a LED? since they're just one frequency of light?

if I am getting this right, we try to knock off the photon's orbitals we reduce the chance of having the photon being attracted to something else due to its charge. when it doesn't have a electron, is it neutrally charged so it doesn't change direction?

when i look at my laser pointers, i noticed that the actual shape of the laser itself is rectangular. why in the world would they/it do that?


by the way, that was an excellant explanation. that said a lot.
thanks for the link Birkeman. that helps also.
i know there's a lot of history and technical information on lasers,
im just trying to get the concept.

 

[Danger]

There still seems to be a lot of confusion about the photon/electron interaction. I'll just walk you through the stages of a ruby pulse laser firing, since that's the one that I'm most familiar with.
To start with, the lasing medium (resonance chamber) is a synthetic ruby crystal rod about 1/4" in diameter and 4 or 5" long. The ends are ground to optically-pure precision, perpendicular to the long axis. One end is completely coated with silver or aluminum to create a total mirror. The other is more thinly coated, so as to be only about 95% reflective.
In the original version, a xenon flash tube was spiralled around the rod. A more efficient one has a straight flash tube lying parallel to the rod, with each of them at one focus of a parabolic mirrored enclosure (shaped like a car muffler). Anyhow, when the flashtube is fired, it pumps a lot of light into the rod.
Some of the photons of that blast hit the valence electrons of chromium atoms in the ruby. Those electrons get punted up to a wider orbital, corresponding to the increase of energy imparted by the photons. Orbitals are set at discrete distances from the nuclei, so an electron can absorb only the amount of energy to precisely raise it one level. Any extra is wasted. That new orbital is not a natural state (it's called 'excited'). In order to drop back down to its ground state, the electron must shed exactly the amount of energy that it absorbed. That means that it has to spit out a photon of that energy, which corresponds to a specific frequency. That's why different substances lase with different colours.
All of the newly produced photons, as well as the original leftovers (non-absorbed ones) from the flash, are playing pinball inside the rod. By sheer probability, some of them end up going back and forth between the two end mirrors. Some hit more electrons, but generally you end up with more and more of them going in the same direction. The random ones continue to create more, and of those a few end up in the parade.
The index of refraction of the partially-silvered mirror is very high, but it's not completely reflective. When the sheer number of photons hitting it gets high enough, a fair number of them manage to get through without being refracted or reflected. Those are the ones that constitute the 'beam'.
In a way, you can think of the rod as a capacitor for light. A lot more goes in from the flash tube than comes out, but it stores the light and manipulates it into something more concentrated. The total amount of of light is low compared to the flash tube, but the intensity of it is huge.

 

[taylaron]

beauty eah.

is it hard to create a very large laser (diameter wize)
that could possibly be used for a car. as to only hit the ground, not shine up and hit the oncoming driver's eyes...
there's the issue of white light...that would be pretty trickey. what about off-white? they could be mounted very low on the car and sine up, at a specific angle to determin how much road you can see.


the whole point of this is to use the uni-directional part of lasers in cars so you don't have the side effefcts to really bright lights.

"If I have seen further, it is by standing upon the shoulders of giants." Sir Isaac Newton (1642-1727)

 

[Danger]

If that's what you have in mind, I'm afraid that you're barking up the wrong bush. To start with, you can't have a white light laser... at least not with current technology. There is a type called 'mode-locked' that can produce more than one frequency of beam at the same time, but you won't likely get a proper blend to create white.
As for the side effects, even your little laser pointer can cause serious eye damage. The kind of thing that you're thinking of would definitely blind anyone who looked at it.
You'd be far better off going with current commercial lighting technology. The legal ones are quite good now, but you don't have to go that route. (My buddy has a pair of 300 watt aircraft landing lights in his semi, and I'm quite partial to the Q-Beam 2,000,000 candle-power spotlights. )

 

[My user name]

powerful lasers

if you want a really potent laser you NEED to manufacture an argon laser. the beam will vaporize right thru steel plate. and talk about beautiful. be careful. these will cut you in half. oh and the beam as far as i know will keep on going. one i made went so far i lost sight of it. i don't know if you know this or not but a pulse laser is stronger than a beam laser.

oh and if you are having difficulty locating a good source of argon gas it is used in electric arc furnaces.

 

[Danger]

True, but it doesn't really address the issue of high-powered headlights.

 

[Danger]

I'm not usually one to resurrect threads this old, but I've just run into something relevant. I stand corrected about not being able to create a 'white' laser.
This month's SciAm has an article entitled 'The Ultimate White Light' by Robert R. Alfano which deals with his creation and use of what he terms a 'supercontinuum' laser. He passes a regular laser through an optical medium that expands its bandwidth to encompass all visible frequencies. Very cool.

 

[berkeman]

Interesting. I'll pick up a copy -- I wonder how that works?