Fluorescence

Fluorescence is the emission of visible light by a substance that has absorbed light of a differing, usually invisible, wavelength. Absorption of a photon triggers the emission of a photon with a longer (less energetic) wavelength. A shorter wavelength emission is sometimes observed from multiple photon absorption. The energy difference between the absorbed and emitted photons ends up as molecular rotations, vibrations or heat. Sometimes the absorbed photon is in the ultraviolet range, and the emitted light is in the visible range, but this depends on the characteristics of the particular fluorescent substance.

Here High Energy Photon is absorbed and low energy photon is emitted