Introduction
This topic covers electromagnetic (EM) radiation: how it is produced, the parameters that describe a wave, the ordered spectrum from radio waves to gamma rays, and the wave-particle duality that lets light and other radiation be described both as waves and as particles called photons. The spectrum decides how radiation interacts with tissue, and this links the topic to imaging and therapy.
Electromagnetic waves and their parameters
An electromagnetic wave is a coupled, self-propagating oscillation of electric and magnetic fields. It is transverse, and it needs no medium. In vacuum it travels at c = 3 \times 10^{8} m/s. The main parameters are:
- wavelength λ, the distance between two successive crests, in metres or nanometres;
- frequency ν, the number of oscillations per second, in hertz;
- speed v in the medium, which is lower than c in matter and depends on the refractive index n, through v = c / n;
- intensity, the power transported per unit area.
The wavelength and frequency are linked by c = \lambda \nu. Since the speed is constant in vacuum, a short wavelength always means a high frequency.