MadSci Network: Physics |
Reply:
>How can we see laser beams fromt the side if all the
>rays are coherent?
>As an aside, why doesn't the air burn up when the
>laser travels thru it?
Sridhar,
Good question! The reason you can see a laser beam from the side, even when it is almost perfectly coherent, isn't because some light isn't coherent and aims for your eyes, but because the (coherent) light hits particles in the air and scatters off, some of which then aims for your eyes. In space (or a laboratory vacuum) no scattered light can be seen. Even in a clean environment, it can be hard to see even a very powerful laser beam. For this reason, sometimes fog is squirted in the path of the laser to make the beam more obvious. This is what they do at rock concerts, for example.
The reason air doesn't burn when laser light travels through it is because the laser isn't powerful enough. Air has a certain "dielectric strength" which measures how much energy it takes to split apart the oxygen and nitrogen molecules that are in it. If the laser beam is more powerful than this (and this is easy to acheive with very-short-pulse-length lasers), then they will indeed "burn" the air. (You can hear a popping sound when the laser pulses and the air is "burnt" at the focused spot of the beam!)
I hope these answer your questions :-)
-Greg Billock
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