- #1
loom91
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Hi,
A few days ago, during a discussion at my chemistry class, I suddenly realized something very fundamentally puzzling about colour. We say that if in a molecule (say a conjugated organic system like beta-carotene) the HOMO-LUMO gap corresponds to a visible frequency of light, we observe that compound to be coloured because that frequency is absorbed and we see the complementary colour.
But consider this: when equilibrium has been established between the ground state and he excited state, the number of molecules getting excited in unit time is the same as the number getting deexcited. This should mean that the same amount of light being absorbed is also being emitted. If this were not true, then molecules would accumulate in the excited state.
This seems to imply that coloured compounds would decolorise after a short exposure to white light. While light does very gradually decolourise substances, it is due to photochemical decomposition and oxidation of dyes rather than reaching equilibrium. What is the explanation? Thanks.
Molu
A few days ago, during a discussion at my chemistry class, I suddenly realized something very fundamentally puzzling about colour. We say that if in a molecule (say a conjugated organic system like beta-carotene) the HOMO-LUMO gap corresponds to a visible frequency of light, we observe that compound to be coloured because that frequency is absorbed and we see the complementary colour.
But consider this: when equilibrium has been established between the ground state and he excited state, the number of molecules getting excited in unit time is the same as the number getting deexcited. This should mean that the same amount of light being absorbed is also being emitted. If this were not true, then molecules would accumulate in the excited state.
This seems to imply that coloured compounds would decolorise after a short exposure to white light. While light does very gradually decolourise substances, it is due to photochemical decomposition and oxidation of dyes rather than reaching equilibrium. What is the explanation? Thanks.
Molu