Professor Lewin revels in his fan mail and in the idea that he is spreading the love of physics. “Teaching is my life,” he said.

The professor, who is from the Netherlands, said that teaching a required course in introductory physics to M.I.T. students made him realize “that what really counts is to make them love physics, to make them love science.”

He said he spent 25 hours preparing each new lecture, choreographing every detail and stripping out every extra sentence.

“Clarity is the word,” he said.

Fun also matters. In another lecture on pendulums, he stands back against the wall, holding a steel ball at the end of a pendulum just beneath his chin. He has just demonstrated how potential energy turns into kinetic energy by sending the ball flying across the stage, shattering a pane of glass he had bolted to the wall.

Now he will demonstrate the conservation of energy.

“I am such a strong believer in the conservation of energy that I am willing to risk my life for it,” he says. “If I am wrong, then this will be my last lecture.”

He closes his eyes, and releases the ball. It flies back and forth, stopping just short of his chin.

“Physics works!” Professor Lewin shouts. “And I’m still alive!”

Chasing rainbows hooked Mr. Boigon, the San Diego florist. He was vacationing in Hawaii when he noticed the rainbow outside his hotel every afternoon. Why were the colors always in the same order?