Why the colour of complexes is the complement of light absorbed? [closed]

Question

From crystal field theory, the splitting of the degenerate $d$ orbitals gives the opportunity for electrons to be promoted, and therefore absorb some light. The colour of the complex is then determined by the light that is not absorbed. However, this doesn't make much sense to me.

In physics, when electron promotion occurs, it is the falling of the electrons back down that gives the colour we see. Hence, when a certain wavelength of light is absorbed, when the electron falls back down again from the higher $d$ orbital, that same wavelength of light should be emitted, so why do we see the complement? Do electrons not fall down?

My intuition of colour from physics seems to contradict crystal field theory.

Answer 1

Your question is not specific to chemical d-complexes.

Energy due photonic excitation is in general usually dissipated by non radiative means (aside of thermal radiation). The topic is covered by general principles of colorimetry, additivity of lights and subtractivity of colors.

The typical examples are organic dyes.

It is common knowledge that dyes of warm colours get "bleached" by sunlight much faster than cold colors like blue. That is because the former absorbs the most energetic photons of near UV, violet and blue light, what causes slow decomposition. Blue colour absorbs low energy photons at red side of visual light spectrum, that is much less effective in photolysis of organic compounds.

As the result, color prints exposed to sunlight gradually shifts their colour to colder hue and finally only somewhat bleached blue remains.

Answer 2

Apparently, you are mixing two independent phenomena ; 1) Absorption of light by any colored substance, 2) Emission of light by luminous substances. Your first paragraphe describes how light is adsorbed by any matter. Your second paragraph describes how light is emitted by a lamp or a fluorescent substance which has received enough energy by a previous phenomena. These two phenomena are not necessarily related to the same color.