In each decade prophets predict the maximum frequency which triode oscillators may ever attain. In each decade the limit is revised upward by a factor of two. The present upper limit for commercial triodes is about 6000 Mc/sec (5-cm wavelength). For each demonstration in physical optics there is a most convenient wavelength. Waves of 12 cm are most convenient for classroom demonstrations of the Fresnel diffraction. For the Fraunhofer and Bragg diffraction 5-cm waves are better. A microwave spectrometer for 5-cm waves is described.