The phenomenon of fluorescence in a fluorescent lamp mainly occurs due to the presence of:

Fluorescent lamp

• A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light.
• Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence.
• An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow.
• A fluorescent lamp converts electrical energy into useful light much more efficiently than an incandescent lamp.
• The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output.