Electromagnetic radiation is different from ionising radiation (which is what we get from radioactive substances), and is commonly just called light in chemistry. Although we usually consider it as a single wave, it is actually composed of perpendicular (at right angles) electric and magnetic fields; hence the term electromagnetic.

Wave nature

We use two basic properties to describe a light wave:

Frequency (ν) is the number of complete waves passing a point per unit time. We normally measure this as the number of oscillations per second, in Hz (s-1).

Wavelength (λ) is the distance between wave peaks (one repeating unit), usually measured in metres (m).

Wave properties

These properties are related by the equation:

$\nu = \frac {c} {\lambda}$

Where c is the speed of light in vacuum (299,792,458 m s-1) – usually a good enough approximation for the speed of light in air.

Particle nature

Atomic-scale objects show both wave-like and particle-like behaviour (wave-particle duality), and the energy of light is carried by particles called photons. We can describe the energy of a photon with the Planck-Einstein equation:

$E = h \nu$

Where E is the energy (in Joules, J), ν is the frequency (in Hz) and h is the Planck constant (6.626 x 10-34 J s). This allows us to link a wave property (frequency) and a particle property (photon energy).

If a moving particle has mass, it must also have momentum. We may describe this using the de Broglie relation:

$\lambda = \frac {h} {mv}$

Where λ is the wavelength (in m), h is the Planck constant (in J s), m is the particle mass (in kg) and v is the particle velocity (in m s-1). Mass multiplied by velocity is equal to momentum, p (with units of kg m s-1). This also allows us to link a wave property (wavelength) with a particle property (momentum).