Electromagnetic radiation includes a spectrum of different waves known as the EM spectrum. These waves can be characterised in terms of wavelength, the distance between two successive peaks (or troughs) of a wave, and frequency, the number of complete wave cycles (oscillations) that pass a given point per second.

This is the electromagnetic spectrum.

The relationship between the frequency and wavelength of a wave is as follows.

$$ c=f\lambda $$

Furthermore, electromagnetic radiation can be described as a wave (it has a wavelength and frequency), and as a particle, and is said to have a dual nature.

The visible spectrum is a small facet within the EM spectrum. Wavelengths of visible light are normally expressed in nanometres ($nm$).

When electromagnetic radiation is absorbed or emitted by matter it behaves like a stream of particles. These particles are known as photons.

→ A photon carries quantised energy proportional to the frequency of radiation.

When a photon is absorbed or emitted, energy is gained or lost by electrons within the substance.

→ The photons in high frequency radiation can transfer greater amounts of energy than photons in low frequency radiation.

The following relationships can be used to determine the energy associated with a single photon.

$$ E=hf $$

$$ E=\dfrac{hc}{\lambda} $$

The energy associated with one mole of photons can be given by the following relationships. The units of energy are often $kJ \space mol^{-1}$ (to convert to this from $J \space mol^{-1}$, the units which the calculated answer will be in normally, divide by $1000$).

$$ E=Lhf $$

$$ E=\dfrac{Lhc}{\lambda} $$

When energy is transferred to atoms, electrons within the atoms may be promoted to higher energy levels.

An atom emits a photon of light energy when an excited electron moves from a higher energy level to a lower energy level.