The energy transitions associated with visible and ultraviolet absorption involve electrons moving from lower to higher energy levels. The lower the amount of energy, the 'redder' the absorption. Blue materials absorb red, so solutions that are blue in color, such as aqueous copper sulfate, are absorbing the red. Wolland offers a very readable account of the electronic spectra of molecules [local] .

Filters which block out certain regions are used in applications from photography to inexpensive spectrometers. Filters absorb most of the light but transmit the desired color. Colors may be dyes printed on a surface like an ink or compounds incorporated in hot glass or polymers. To excite molecules selectively in the region from 400 to 600 nm a filter which absorbs light from 600 to 800 nm can be placed in the light path. White light incident on this filter will be color shifted to blue. Colored transparency sheets make easy filters for demonstrations or student experiments.