Wave Optics

This huge radio dish nestles in a natural canyon at Arecibo, Puerto Rico. Its apparently solid surface is actually a scaffolding that hangs like a suspension bridge. It is a telescope that images the universe using radio waves rather than visible light, and it is also used to search for artificial radio signals from intelligent beings on other planets. Why does it have to be so huge? It's not mainly a question of picking up weak signals. The dish's sensitivity is overkill for most jobs, and for example it would have easily been able to pick up signals from a species on the other side of the galaxy that were no more intense than the ones humans themselves have transmitted out into space. (This is assuming that it was tuned to the right frequency at the right time, and that the signals came from within the limited field of view it sweeps out as the world spins.) No, the reason it has to be so big is a matter of wave optics. To make the antenna select signals from only a certain direction in space and reject those coming from other angles, it must be as large as possible compared to the wavelength of a radio wave.

Electron microscopes can make images of individual atoms, but why will a visible-light microscope never be able to? Stereo speakers create the illusion of music that comes from a band arranged in your living room, but why doesn't the stereo illusion work with bass notes? Why are computer chip manufacturers investing billions of dollars in equipment to etch chips with x-rays instead of visible light?

The answers to all of these questions have to do with the subject of wave optics. So far this book has discussed the interaction of light waves with matter, and its practical applications to optical devices like mirrors, but we have used the ray model of light almost exclusively. Hardly ever have we explicitly made use of the fact that light is an electromagnetic wave. We were able to get away with the simple ray model because the chunks of matter we were discussing, such as lenses and mirrors, were thousands of times larger than a wavelength of light. We now turn to phenomena and devices that can only be understood using the wave model of light.