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Visible light is electromagnetic radiation, or light energy, with wavelengths between 400 nm (violet) and 700 nm (red), the sensitivity of the human visual system. Plants capitalize on the same range of wavelengths for the process of photosynthesis, so the 400–700 nm range is also known as photosynthetically active radiation (PAR). Visible light is a small segment of the total spectrum of electromagnetic radiation, which ranges from very high‐energy, short‐wavelength, gamma rays to the longer‐wavelength, lower‐energy radiation of radio waves (Figure 1.21).

Light behaves as a particle (photons or quanta) and as a wave with a characteristic frequency and wavelength. The waves oscillate at 90° to the axis of movement and are called polarized if the photons have the axis of oscillation in the same plane. For normal light, the axis of oscillation is random between 0° and 180°. Polarized light is created by passing through a polarizing filter, by reflection off a surface (such as the surface of the ocean) or by scattering from small particles (Withers 1992).

The energy of a photon of light is an inverse function of its wavelength, which is expressed as:

(1.13)

where E is the energy of a photon (joules) – joules (J) are the SI unit for energy = 0.24 calories, h is Planck’s constant (6.626 × 10⁻³⁴ J s), υ is frequency (s⁻¹), λ is wavelength (m), and c is the speed of light (2.998 × 10⁸ m s⁻¹)

Figure 1.21 Total spectrum of electromagnetic radiation.

The equation may be rearranged:

(1.14)

where the energy of a photon of light varies from about 170 kJ mol⁻¹ in red light to 300 kJ mol⁻¹ in violet light. A mole is Avogadro’s number (6.022 × 10²³) of photons.