|What kinds of lasers are used at laser demonstrations? Why and how do they get different colors? How do you see the actual beams?
Most of the visible lasers used in light shows are gas lasers: tubes with gas discharges in them that are arranged to produce laser light. The most common gases used in these tubes are argon and krypton. Argon lasers produce green and blue light very nicely, while krypton lasers are best for intense red light. The colors come from the structures of the atoms themselves; the energies of their various electron orbitals. To see the beams, something must scatter the light. If the lasers are intense enough, Rayleigh scattering from the air is enough to make the beams visible. However, a little mist added to the air helps a lot.