*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.

Electromagnetic radiation

**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:

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*:

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*:

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).

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