White light can be dispersed by a prism, which resolves a beam of white light into its colored components, the spectrum. Visible light is electromagnetic radiation within a wavelength range of about 410 nm (nanometers) to about 770 nm. The various spectral colors may be characterized by their wavelengths within this range.

An object that reflects only the part of white light between 540 nm and 600 nm will appear yellow. Yellow light may also be generated by combining green and orange-red light (the colors adjacent to yellow in the spectrum) or by combining all colors except blue. Blue is called the complementary color of yellow; the other colors also have complements. Colored light mixed with light of its complementary color appears white.

The actual color sensation produced by an object is determined by a combination of the composition of the incident light and the object's reflective properties.  An object illuminated by blue light can, of course, reflect only blue light.  The color of the object will then be observed only as shades of blue or black.  For example, yellow and orange objects reflect almost no blue light and, under these circumstances, will appear black.

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