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Two years after the invention of the daguerreotype, John William Draper (1811-1882) recognised that in every chemical change in a substance caused by light. Light rays of a definite wavelength are absorbed and that it is this absorption which produces the photochemical change.

Stokes, employing fluorescent substances in 1852 found that quartz transmits most ultraviolet rays which led Crookes (1854) to the spectrography of the ultraviolet region with the wet collodion process.

In 1901 Max Planck (1858-1947) demonstrated that the absorption and emission of light, which is of a photoelectric nature, takes place in so-called quanta or packets of energy.

Following on from Planck, in 1905, Albert Einstein (1879-1955) showed that radiation exists in packets in all circumstances and gave the name ‘photons’ to the free-travelling quanta of light.

When light is absorbed an electron or electrons move to higher energy levels. This increases the energy level of the molecule. 

The 1st Law of photochemistry is that no photochemical (or subsequent photobiologic) reactions can occur unless radiation is absorbed. 

Absorption of light involves the transfer of energy, hv, from light to individual molecules in the chemical.Substances all have their own absorption spectrum.

The longer the wavelength the less energy. The shorter the wavelength the more energy. This is the main reason why ultraviolet and blue light are more likely to cause fluorescence because they have a higher energy.

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