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Q & A: non-laser

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Most recent answer: 10/22/2007
Q:
Several years ago, some of us (my friends and I) were trying to construct a low level, inexpensive laser. We settled eventually on using mirrors as the cheapest, though it would take some mathematical equations to get the angles right. We never got around to building it, but I would like to know if the idea would work. The light entered through a one-way mirror in the top. (I don’t know if that was necessary, as if the angle was right, it shouldn’t come back out.) Next it hit an angled mirror, off of which it shot straight forward. On its way out, it would pass through three focusing glass plates, like those used in microscopes. It would taper as it got down to the end, where it would come out through a hole. Assuming all of the calculations are precise and everything is in place, would this work, and if not, why? Here’s a little picture of it, but I don’t know if it’ll show up right: Light || \/ | | | | | |________________________ | | | | | | |\_ |---------| | | | \ The Laser | \_|==========|----------||-------\_________ |____\_________|_|________|__|______/ Yeah, like that.
- The Builder of Things (age Old)
Indianapolis, IN, US
A:
Dear Builder- the contraption you've described is basically a telescope, or something like it. Light comes in over a wide area, and gets focused down to a smaller area. There's a basic tradeoff in any device like this. If you focus down to a very small area, then the light will be spread out over a big range of directions. If you want the light to travel in a small range of directions, you'll need a big wide beam. The only way you can get a narrow beam with a narrow range of directions is by throwing out most of the incoming light, say when it hits the plate with the little hole. Incidentally, the one-way mirror has a major drawback- nonexistence. The things that get called one-way mirrors are just partially reflecting mirrors, with one side kept light and the other kept dark.

A laser works in a completely different way- one that only makes sense via quantum mechanics. Something in the laser (often, individual atoms) has a set of special states each with a specific value of energy. One of the "excited states" (states with more energy than the "ground state", which has the least) has no easy way to dump its energy and fall to the ground state. There's some way of pumping energy in excite atoms to higher states so that they trickles down to that special state. Then they release energy mainly when triggered to do so by their environment- a process called stimulated emission. What makes the light unusual is that the stimulated light comes out at the same frequency, direction , etc as the light that stimulated it. So the light can come out in the narrowest range of directions and positions allowed by basic physics. There's still a trade-off between getting the beam narrow at some point and getting it to travel a long way before it spreads out, but the beam is much closer to being an idealized ray than anything you can get from ordinary light sources.

Mike W.

(published on 10/22/2007)

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