The Radio Spectrum

At every point in space, there is something we call the "electric field." It's a tiny vector, a quantity with a magnitude and a direction. If you want to measure it, just put an electron at rest at that point, and watch it start moving. The direction and size of its acceleration (over and above what we get from gravity) is proportional to the electric field. Typically, if you watch closely enough, you'll see our little electron jiggle back and forth like mad. That's because the electric field doesn't just sit there; we are surrounded by an extraordinary superposition of all kinds of electromagnetic waves, pushing by us with different amplitudes and directions and frequencies. If you build the right type of gizmo with an appropriate collection of electrons, you can pick out just a single wavelength from amidst the cacophony. Voila! You are listening in on the electromagnetic spectrum. ...