Nature of Light

Light is a form of electromagnetic radiation with a dual character: quantum theory describes it as a stream of discrete particles called photons, each photon carrying one 'quantum' (packet) of light energy, while wave theory describes it as travelling in waves of different wavelengths, which is what gives light its different colours. The human eye can detect electromagnetic radiation only across a narrow band, roughly 390 nm to 730 nm, called visible light, which sits between the ultraviolet and infra-red regions of the wider spectrum.

Every photosynthetic pigment absorbs different wavelengths of light to different degrees, and plotting how much light a pigment absorbs at each wavelength produces a curve called the absorption spectrum. A separate but related graph, the action spectrum, plots the actual rate of photosynthesis produced at each wavelength of light. For chlorophyll a and b, the absorption spectrum shows clearly that more light is absorbed at the blue-violet and red wavelengths of the visible spectrum than in the green-yellow middle; the action spectrum for photosynthesis follows an almost identical shape, confirming that the wavelengths chlorophyll absorbs best (red and blue) are the very wavelengths that drive photosynthesis fastest. Because …