A form of energy necessary to see. Visible light is that part of the spectrum that produces sensation of sight in the human eye. The radiation wavelengths in this visible range are very small and are usually expressed in either millimicrons or nanometers. The values given for the limits of the visible- light wavelength range depend on the textbook you are reading, but are around 380-760 nanometers or 400-750 nanometers. The rays that cause us to see violet have the smaller, shorter wavelength, around 380 nanometers or millimicrons. Each color of the spectrum – blue, green, yellow and orange – have a range of wavelengths. Red has the highest range, at around 660-750 nanometers.