When demonstrating lasers to children, I like to delight them with the simple chalk dust scattering demonstration familiar to many of us. I direct a beam across the room, using caution to make sure the beam doesn't strike anyone in the eye, and have a volunteer walk along the beam while patting dusty erasers together (see image on page 63). As the children observe the scintillating beam, I ask them to tell me what's happening. Why does the chalk dust allow them to see the beam? What's wrong with all those sci-fi movies where we see brilliant visible laser beams zipping through the vacuum of space?
by Michael E. Knotts