What is the principle of fluorescence?

• A. Emission of light by a substance after absorption; a form of luminescence; emitted light has longer wavelength than absorbed radiation.
• B. Absorption of light by a substance without emission.
• C. Production of light by chemical reaction near a surface.
• D. Scattering of light.

Answer: (A)

Fluorescence is the emission of light by a substance after it absorbs light, and it is a form of luminescence. After absorption, the molecule is promoted to a higher electronic state, but it relaxes quickly to the lowest vibrational level and then returns to the ground state by emitting a photon. The emitted photon has lower energy than the absorbed one, so its wavelength is longer—a phenomenon called the Stokes shift. This rapid, photon-emitting process distinguishes fluorescence from simple absorption (no emission), from light produced by a chemical reaction (chemiluminescence), or from light simply scattered by a material.