Electromagnetic radiation with wavelengths between 400 nanometers and 750 nanometers. Electromagnetic radiation within this range can be detected by the human eye. Colors depend on the wavelength lengths; a short wavelength (the 400 nm side) looks blue and a long wavelength (the 750 nm side) looks red.

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From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.