# Q & A: Stopping a Laser

Q:
Do laser beams cancel each other out when they cross,how, and how else can you stop them.
Thank you
- Terry Wilson
California
A:
Lasers are made up of light waves. If you’ve been to a beach or watched a movie with a beach in it, you’ll see how the water has waves that stretch to the left and right a long ways. That is similar to a laser. The light waves and all lined up together and move together.

Now, if you have 2 lasers and you point them at each other, something will happen. You’ll will have 2 different sets of waves moving in opposite directions. Here’s a neat thing about light waves...they add together. So when the high points of both waves are at the same spot, the result is even higher. On the other hand, if a high point of one wave and the low point of the other are at the same spot, they cancel each other out. So what happens is there will be some spots where the light is stronger and some spots where there is no light. And those might even move depending on your lasers.

The best way to stop a laser is to use a mirror to send it away or put some object in the way. The object will absorb the light from the laser much like your clothes absorb some of the light from around you.

Good question, Terry.

Actually, when two laser beams (or any other kinds of light for that matter) collide, they do not stop each other. In fact, usually they behave as though the other is not there at all. This is because light is a wave, and usually waves cross each other without interacting.

As for how to stop a laser, you simply put something in front of its path. Lasers cannot be stopped by other light, but they can be stopped with matter. So if you shine a laser pen on the wall, the laser will not go through the wall because the matter has stopped it and scattered it.

Some lasers are powerful, and can burn the things they touch, but if you put enough stuff in front of it, the laser will be stopped.

I hope this answers you questions.

-math dan

(published on 10/22/2007)

## Follow-Up #1: Laser burning through a mirror

Q:
if you get a powerful enough laser, can it burn through a mirror, or will it always be reflected?
- Joel Rushton (age 16)
Palatine, Illinois, USA
A:
In principle you can burn through any mirror if the laser is strong enough.   The absorption coefficient of  the very best mirrors is of the order of 1 part per million.  So compared with an object that has an absorption coefficient of 1, like a black disk, then you would need a laser with 1 million times the amount of power.

LeeH

(published on 05/30/2010)