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Asked by Mason The Amazing :) to Andrew, Hina, Ian, Kathryn, Leah-Nani, Xu on 14 Jun 2018.
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Kathryn Burrows answered on 14 Jun 2018:
Not very, in fact, this is kind of undesirable. Ideally laser beams would only be seen by looking head on. This is usually the case when the laser beam is travelling through a vacuum (very little air/gas). It is often only interactions between the laser beam and the air that cause light to be sent out sideways from the main beam and seen, but the brightness of this is much weaker than the main beam itself. If your laser beam was very very strong, and you had lots of gas particles to collide with, this could be quite bright.
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Leah-Nani Alconcel answered on 15 Jun 2018:
Ideally you wouldn’t be able to see a laser beam travelling in vacuum from the side at all. In the lab, however, you usually can, and they’re pretty bright. It depends on the class of laser, but most of the ones you use in labs are quite powerful and you need to wear special glasses to block the light coming from the sides of the beam.
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Andrew Margetts-Kelly answered on 20 Jun 2018:
Not very bright compared to if you looked at it head on, but you can see it. (but never do that btw)
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In space you would be forgiven for thinking that you wouldn’t see anything. But what if I told you that light doesn’t actually travel in straight lines; light actually take every conceivable path through space on it’s journey (but don’t say that in your GCSE exam though, it’ll melt the marker’s brain and you won’t get any marks).
Even the most collimated light (that’s the fancy word meaning all going in the same direction), has some of the energy travelling sideways, in fact in every direction, and these effects are detectable in real world experiments. Over a long distance, like a millions miles, the beam will spread out and be quite fat. If you stuck your head into the beam looking across it you’d see a detectable amount of light if the beam was bright enough to start with.
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If the laser is travelling through air, it’s a different story. Even if there is no dust to reflect the light in the beam (this causes the speckle look of laser beams from the side) the gaps between the gas molecules will diffract some of the light and it will then go sideways. It’s called Rayleigh Scattering, which is also the reason why the sky is blue. This is why you can see a laser beam from the side.
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