explain gruthers drappers law
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The Grotthuss–Draper law (also called the Principle of Photochemical Activation) states that only that light which is absorbed by a system can bring about a photochemical change. ... The law was first proposed in 1817 by Theodor Grotthuss and in 1842, independently, by John William Draper.
In general, a small, simple molecule luminesces in the ultraviolet, and a more complex one emits near the blue-violet end of the visible spectrum. Dye molecules, on the other hand, may emit throughout the visible region, including the red end. The ground electronic state of most molecules is a singlet state. Usually, therefore, the optically allowed emission, or fluorescence, is from the lowest excited singlet state to the ground state. The lowest triplet state of the molecule lies somewhat below the excited singlet. Light emission from this triplet state is forbidden by the quantum-mechanical selection rules, but it does occur by default when other processes are even less probable. Such emission is called phosphorescence. It is relatively weak, slow, and shifted toward longer wavelength. Triplet states may be produced from higher singlets by processes called internal conversion and intersystem crossing. The states may also be produced in excitation from the ground state by impact of relatively slow charged particles, such as electrons.
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