How it happens that some kind of laser beams are visible?

Answer 1

Stolen from the Wiki article...

The representation of lasers in popular culture, especially in science fiction and action movies, is often misleading. Contrary to their portrayal in many science fiction movies, a laser beam would not be visible (at least to the naked eye) in the near vacuum of space as there would be insufficient matter to scatter off.

In air, however, moderate intensity (tens of mW/cm²) laser beams of shorter green and blue wavelengths and high intensity beams of longer orange and red wavelengths can be visible due to Rayleigh scattering. With even higher intensity pulsed beams, the air can be heated to the point where it becomes a plasma, which would also be visible. This would also cause rapid heating and explosive expansion of the surrounding air, which would produce a popping noise analogous to the thunder which accompanies lightning. This phenomenon is also capable of causing a retro-reflection of the laser beam back into the laser source possibly damaging its optics. When this phenomenon occurs in certain scientific experiments it is referred to as a "plasma mirror" or "plasma shutter".

Answer 2

Lasers act the same as all light, only they concentrate light into a beam. Your question is a lot like asking why the sky is blue.

When you look into space, light is still traveling all around you, but it is still black. So what makes space different from standing here on Earth? The atmosphere serves to scatter and reflect light off molecules, which make the sky appear blue rather than black.

The same idea is true for lasers, only the distance the laser beam travels is generally a lot shorter than the distance sunlight takes coming to earth through the atmosphere, so you need a much higher concentration of molecules to make the laser beam visible.

So really, it has nothing to do with the laser, just how dusty or foggy the air is.

Answer 3

Laser beams in general are invisible.
This is because they are collimated and do not scatter.
However sometimes the air is filled with tiny particles like water or dust.
As the laser beam moves through this dust it is scattered and a small fraction of the light is reflected into our eyes so that it appears that we are seeing the beam.
In reality most of the beam does not scatter and continues on a straight path until it attenuates or is absorbed.

Answer 4

if you mean how we can see the beam in the air, its becauce the light hits various particles in the air, dust, smoke whatever. You can´t see a beam in clean air.