An alternate title for this talk might be “Walter’s World of Waves.” First, Lewin methodically explores how water and sound move in traveling waves. With a pan of water and a tuning fork, Lewin demonstrates concepts of oscillation, amplitude and velocity. He deploys equations for determining wavelengths and measuring frequency. By tapping water, he illustrates constructive interference, where “waves support each other” and achieve maximum amplitude. Then Lewin synchronizes two water waves so that the peak of one wave arrives precisely at the valley of the other, and “the two waves annihilate each other.” The same principles apply to sound. Lewin broadcasts tones from loudspeakers, and gets heads bobbing as listeners try to detect if two sound waves are capable of destroying each other to produce silence. If experiments like these show how sound and water move in waves, can he similarly demonstrate that light exists as waves? He replicates with a red laser an experiment from 1801 that yielded such proof, “that light plus light can give darkness.” When he aims his bright light at two openings, Lewin gets areas of darkness and areas of light, evidence of both destructive and constructive interference. But while he offers definitive proof that light travels in waves, Lewin concludes that it also exists as particles, “a great mystery of physics that causes me and others sleepless nights.”