electromagnetic wave

An electromagnetic wave is a ripple of energy that carries itself through space at the speed of light — and it's everywhere around you right now. The light you're reading by, the radio in your car, the signal from a Wi-Fi router, and the X-rays at a clinic are all the same kind of wave, just stretched to different sizes.

What's rippling? Two invisible things, working as a team: an electric field and a magnetic field. As one wiggles, it stirs up the other, which stirs up the first again — so the wave keeps regenerating itself as it goes, needing nothing to travel through. That's why sunlight crosses the empty vacuum of space to reach us.

The big difference between, say, a radio wave and an X-ray is just the wavelength — how long each ripple is. Long, lazy waves are radio; short, tight ones are X-rays and gamma rays; visible light sits in a thin band in between. In the 1860s James Clerk Maxwell wrote the equations showing all of this is one phenomenon, and even predicted these waves travel at light speed — revealing that light itself is electromagnetic.