Blacklight Fluorescence: What Glows and Why

UV Blacklight

UV intensity 70%

Material

Photon energy — absorbed UV → emitted visible

Absorbed (UV)

365 nm

Emitted (visible)

— nm

How it works

A fluorescent material absorbs a high-energy, short-wavelength UV photon, kicking an electron up into a higher energy state. Before it drops back down, the electron loses a small amount of that energy as heat (molecular vibration), so when it finally falls back and emits a photon, that photon carries less energy — and therefore a longer wavelength — than the one that was absorbed. This unavoidable energy loss is called the Stokes shift, and it's exactly why a blacklight — UV, invisible to our eyes — can make certain materials emit visible colored light: we see the emitted photon, not the absorbed one.

Which visible color comes out depends entirely on the material's electronic structure. Quinine in tonic water fluoresces blue; many scorpion cuticles and minerals such as willemite fluoresce green under UV thanks to trace impurities and conjugated ring structures; some minerals fluoresce orange or red; and the optical brighteners in laundry detergent absorb UV and re-emit blue-white light, which is why "clean" white fabric glows under a blacklight.

Drag the lamp to move it · drag the small ring to aim it