SQA Advanced Higher Chemistry

## Waves and photons Electromagnetic radiation can be described as a wave, with a wavelength and a frequency, and it can also be described as a stream of particles called **photons**. This is its **dual nature**. The wave description accounts for how radiation travels and spreads out; the particle description is needed whenever radiation is absorbed or emitted by matter, because energy is then exchanged in whole photons and never in fractions of one. The wavelength $\lambda$ is the distance between two adjacent crests and is measured in metres, although wavelengths of visible light are normally quoted in nanometres ($\pu{1 nm} = \pu{1e-9 m}$). The frequency $f$ is the number of waves passing a point each second, in hertz ($\pu{Hz} = \pu{s-1}$). All electromagnetic radiation travels at the same speed $c$ in a vacuum, about $\pu{3.00e8 m s-1}$, so wavelength and frequency are linked by $$c = f\lambda$$ A longer wavelength therefore means a lower frequency. The **electromagnetic spectrum** is the full range of radiation arranged in order of wavelength. From the longest wavelength to the shortest it runs through radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Visible light covers roughly 400 nm (violet) to 700 nm (red). ## Photon energy A photon carries a quantised amount of energy that is proportional to the frequency of the radiation: $$E = hf \qquad\text{or}\qquad E = \frac{hc}{\lambda}$$ Here $E$ is the energy of one photon in joules…