Define fluorescence.

• A. The phenomenon of absorbing incoming radiation and re-radiating it as lower-energy radiation.
• B. Emission of high-energy gamma rays by unstable nuclei.
• C. Absorption without re-emission of energy.
• D. Scattering of light without energy change.

Answer: (A)

Fluorescence is when a substance absorbs photons and then re-emits light as it returns to a lower energy state. After absorbing energy, the excited electrons lose some of that energy through non-radiative processes (like vibrational relaxation) before emitting a photon, so the emitted light has lower energy (longer wavelength) than the absorbed light. In X-ray fluorescence, the incident X-rays eject inner-shell electrons and the emitted photons are the characteristic X-rays specific to the elements present, still following the same idea of re-emission after absorption. This distinguishes fluorescence from merely absorbing energy without re-emission, from scattering with no energy change, or from nuclear gamma emission, which is not related to electronic transitions in the material.