Alternating Current

Electromagnetic waves are transverse oscillations of an electric field and a magnetic field, and they are produced by accelerating charges. The electric and magnetic vectors are mutually perpendicular and both are perpendicular to the direction in which the wave travels, which is what makes the wave transverse rather than longitudinal. Maxwell is credited with the theory, and the entire spectrum from radio waves to gamma rays is the same phenomenon at different frequencies.

In a vacuum every electromagnetic wave travels at the same speed, $c = \dfrac{1}{\sqrt{\mu_0 \varepsilon_0}} \approx 3 \times 10^{8}\ \text{m},\text{s}^{-1}$, whatever its frequency. Inside a material the speed is set by the permittivity and the permeability of that medium. Ordered by increasing frequency the regions run radio, microwave, infrared, visible, ultraviolet, X-rays and gamma rays. Visible light occupies the narrow band from about $400$ to $700\ \text{nm}$. Gamma rays have the highest frequency and the greatest penetrating power and are used to kill bacteria and insects in food; ultraviolet rays cause sunburn, blindness and skin cancer and also sterilise water and surfaces; infrared is emitted by warm bodies. Sound waves are not electromagnetic, so they do not travel at the speed of light.