Basic spectroscopy

Light is most strictly defined as that part of the spectrum of electromagnetic radiation detected by the human eye. However, the term is also applied to radiation just outside that visible range, e.g. ultraviolet (UV) and infrared (IR) 'light'. Electromagnetic radiation is emitted by the sun and by other sources (e.g. an incandescent lamp) and the electromagnetic spectrum is a broad band of radiation, ranging from cosmic rays to radio waves (Fig. 26.1). Most chemical experiments involve measurements within the UV, visible and IR regions (generally, within the wavelength range 200-1000 nm).

Radiation has the characteristics of a particle and of a vibrating wave, travelling in discrete particulate units, or 'packets', termed photons. A quantum is the amount of energy contained within a single photon (it is important not to confuse these two terms, although they are sometimes used interchangeably in the literature). In some circumstances, it is appropriate to measure light in terms of the number of photons, usually expressed directly in moles (6.02 × 10²³ photons = 1 mol). Alternatively, the energy content (power) may be measured (e.g. in W m⁻²). Radiation also behaves as a vibrating electrical and magnetic field moving in a particular direction, with the magnetic and electrical components vibrating perpendicular to one another and perpendicular to the direction of travel. The wave nature of radiation gives rise to the concepts of wavelength (λ, usually measured in nm), frequency (ν, measured in s⁻¹, but often recorded in hertz, Hz), speed (c, the speed of electromagnetic radiation, which is 3 × 10⁸ m s⁻¹ in a vacuum), and direction. In other words, radiation is a vector quantity, where:

⇒ Equation [26.1]

c = λν

Sometimes, it is necessary to rearrange the equation such that:

⇒ Equation [26.2]

ν =

c λ

Fig. 26.1 The electromagnetic spectrum.