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زمینشناسی و معدن::
فوتوشیمیایی
ft is an unfortunate paradox that the colours we admire are the result of the absorption of light and that we obviously require light to observe these colours; but it is exactly this absorbed light that can produce fading and other photochemical degradations, so that, in time, the colour and even the specimen itself can be completely destroyed.
There are some additional colour alterations derived from transition metal impurity valence changes, either in ligand field (cause 5) or charge transfer (cause 7) environments, and some based on organic compounds (cause 6); all of these are discussed in Section 23, together with some general comments on photochemical changes.
There are few general principles of utility in * b, ing about the photochemical changes produced light or ultraviolet radiation.
Last, raising the temperature speeds up the photochemical effects, as with any chemical reaction.
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