MadSci Network: Physics
Query:

Re: If laser is coherent, how can we see the beam from the side?

Date: Wed Jan 14 10:44:22 1998
Posted By: Greg Billock, grad student,Caltech
Area of science: Physics
ID: 884350820.Ph
Message:

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


Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.



MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci


MadSci Network, webadmin@www.madsci.org

Page generated by MODERATOR_1.2b: Tools for Ask-An-Expert websites.
© 1997 Enigma Engines for a Better Universe: We are forever combustible, ever compatible.