Simple demonstrations can illustrate profound concepts in physics, Nobel-prize winning physicist Edward M. Purcell--who called the overhead projector "the greatest invention since chalk"--told a small crowd in the Science Center yesterday.

Using a tennis racquet, a cardboard box and an overhead projector, Purcell, Gade University Professor Emeritus, demonstrated that light is transmitted in waves rather than particles, and other basic theories of physics.

Purcell also showed how a rocket launches and what the gas argon would like like from the viewpoint of a molecule using polaroid transparencies and an overhead projector.

"I have never seen anything change a teaching situation as drastically and abruptly as the overhead projector," Purcell said to the group of teachers in the last three presentations in the Professional Training Series sponsored by the Harvard-Danforth Center for Teaching and Learning.

"You have to teach physics to understand it," Purcell said. "Teaching has been a central part of my intellectual life as a scientist," he said, adding that he thought it was a myth that research scientists are poor teachers.