intellegent-
The info below was cut-and-pasted from a discussion about lasers.
It contains a reasonably concise explanation of what I tried to describe above.
Why does the laser not create a beam of light that you can see as it travels through the air to its destination (like a flashlight)?
You can only see light travel across a room if something in the air scatters that light toward you. If there is dust, smoke, or mist in the air, you will see that light pass through it. You will see a flashlight beam scattered by these particles and you will also see a laser beam. In that respect, the two kinds of light are very similar. Some laser beams are so intense that the Rayleigh scattering (the scattering that creates the blue sky) is strong enough to make the beams visible even in perfectly dust-free air. The beams shown in class are not that strong and would only be visible if something in the air scattered their light toward your eyes.