Fluorescence
The rapid absorption of light at a shorter wavelength followed by immediate emission of light at a longer wavelength.
Governing formula
h ν_emitted < h ν_absorbed (Stokes Shift)
SI unit
Nanoseconds (Lifetime)
In depth
Involves spin-allowed radiative transitions (singlet-to-singlet). Non-radiative vibrational relaxation causes emitted photons to have lower energy than absorbed photons (Stokes shift). Emission stops almost instantly (~10⁻⁸ s) when excitation light is removed.
Examples in the real world
Fluorescent highlighters, UV security marks on banknotes, and green fluorescent protein (GFP) in cellular biology.