UV light, which also causes sunburn in summer, provides enough energy to stimulate the quinine molecule. This is what gives the tonic water its bluish glow when exposed to UV light.
In this case, we understand excitation to mean that an electron of the molecule is lifted from its location (= orbital) into an energetically higher location.
However, this excited state does not last very long and said electron „falls“ back into its original location. Light is emitted during this transition, but some energy is also released in the form of heat. It follows that the emitted light cannot have the same energy as the incident UV light (otherwise an inexhaustible source of energy would have been found). The initially invisible UV light is therefore emitted with a little less energy and according to the formula above, the new wavelength must therefore be shifted into the visible range – we see a blue light! The phenomenon is called fluorescence. In our case, this also takes place with relaxation, since part of the energy is converted into heat, for example. The processes can also be illustrated using a Jabłoński diagram (see above).