An action spectrum is a representation of the rate of physiological activity plotted against the wavelength of radiation. It shows the efficiency of electromagnetic radiation in producing a photochemical reaction against the wavelength of the light used. An action spectrum demonstrates which wavelength of light is most effectively used in a given chemical reaction and it also indicates the wavelength of light that plants can use for photosynthesis.
Some reactants are able to utilize specific wavelengths of light more effectively to complete their reactions. The pigment molecules in photosynthetic organisms absorb specific wavelengths of light. For instance, chlorophyll is much more efficient at using the red and blue spectra of light to cause photosynthesis to proceed at the fastest rate. Green is not absorbed, and it is reflected instead. Chlorophyll, the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green. The action spectrum graph would therefore show spikes above the wavelengths representing red and blue colors.
The order of colors is determined by light wavelength. Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the redder the color. Similarly, the shorter wavelengths are toward the violet side of the spectrum. Wavelengths longer than red are classified as infrared, while those shorter than violet are ultraviolet.