laser beam invisible?

[devayaani]

Thanks in advance for explaining the mechanism of the optical phenomenon that is troubling me. I have seen in many ( thrillers/spy films) films the invisible laser beam at security points made visible by sprinkling some sort of dust by the person who is trying to avoid the laser-beam triggering an alarm system.
If laser is nothing but light (I mean the same electromagnetic wavelength as light but only the waves travelling in phase and of only one colour) why it cannot be seen without the use of the dust sprinkling? Thanking you

[Mortartube]

They often can in a darkened room because dust in the beam reflects the light out of the side of the beam but in a clean room such as a museum holding items in a controlled atmosphere there is virtually no particles to pass through the beam and so it is almost invisible so adding dust to the atmosphere makes it more visible.

The light in a laser beam is directional hence the beam effect instead of an all round glow like a conventional lamp.

[fo3nix]

Think of it this way:

Take a laser pointer, and point it at a wall.

You'll see a dot on the wall (hence, why they're popular for presentations). Why do you see that dot?

It's because the light is going from the laser pointer, hitting the wall, and then some of that light is reflected back to your eye.

Why don't you see a continuous beam? Because there isn't much reflecting the light between the pointer and the wall, so there's isn't much chance of reflected light getting to your eye.

If you then massively increase the number of particles between the laser pointer and the wall, there's no much more chance of the laser beam hitting some of these particles between the pointer and wall, and bouncing to your eye.

Hence, you can get these beams of laser light by using some sort of spray, or even just using the laser beam in a very foggy area.

[devayaani]

Thank you both Mortartube and fo3nix for the prompt answer and clarifying my doubt.

[rojash]

You might like to bear in mind also, that the reason lasers are used for security systems in thrillers, is that they are so easy to overcome, and attractive to look at. A simple PIR detecor from B and Q is a far more effective intrusion alarm trigger.

[Bert]

Although the question has been answered effectively and devayaani seems satisfied, I think that the simple explanation has been missed. You can only see light when it enters your eye. Hold your hands out in front of you. There is light (from the sun or whatever) bouncing off your left hand and hitting your right hand (it doesn't have to be a laser beam), but you cannot see it, because none of it is entering your eyes. You cannot see light 'from the side'.

[devayaani]

Thanks Bert.
Yes, it is an interesting explanation.
thanks again.