Electromagnetic radiation

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

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

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).

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