• Question: when we fire a strong laser, surely the ray of light would disperse so why is it said that a laser will fire straight on a non stop line?

    Asked by mantha to Adam, Catherine, Karen, Leila, Nazim on 14 Mar 2012.
    • Photo: Adam Stevens

      Adam Stevens answered on 12 Mar 2012:

      Even the strongest lasers disperse as they travel, but light will travel in a straight line until it hits something.

      The dispersion comes from the fact that the laser aperture isn’t infinitely small – there’s no way we can stop that!

      So the laser will disperse slightly along the straight line, but it is still essentially travelling straight.

    • Photo: Leila Battison

      Leila Battison answered on 12 Mar 2012:

      Until all the light has dispersed, the laser will indeed carry on going straight.

      The amount of time it takes for the light to disperse will depend on what it is going through. If there are lots of particles (like in dust, or fog) then the light waves will get bounced off in all directions, and the beam of light will disperse a lot faster. But a laser in the vacuum of space will be able to go further because there is less to scatter it.

    • Photo: Nazim Bharmal

      Nazim Bharmal answered on 14 Mar 2012:

      Lasers have a special property which is called coherence, which basically means that all the light “prefers” to travel together as much as is possible. A the light from a bulb or LED has very little coherence, so the light “prefers” to spread out as much as possible. That is basically why the light from a laser travels together in the direction that it is pointed.

      In the atmosphere, some light does hit the molecules of gas, any dust (which is the same dust that makes cars dirty), drops of water (like fog or clouds), and in fact anything. When it does that, it looses coherence and that bit can go in all directions. Which is why when we see the light in a laser beam, we actually see what the laser beam has lost. In a completely empty place, like deep space, you couldn’t see a laser beam.

      Then there is also dispersion, like Adam says. Even in deep space, the laser beam will spread out and eventually be lost. But that could take hundreds of years, by which time the original laser device will probably be long gone!

Comments