A type of light — or electromagnetic wave — which is invisible to the naked eye. X-Rays are much more energetic than the light we see. They can penetrate skin very easily, for example. In the doctor's or dentist's office, X-Rays are detected on a photographic plate, allowing us to see inside the body. Like all other forms of electromagnetic radiation, X-Rays travel at roughly 300,000,000 meters per second (186,000 miles per second).