What produces the light from a fluorescent tube?

• A. Heated Phosphor Coating on the Inside of the Glass
• B. Mercury Vapor
• C. The Filament Heats the Tube
• D. External Light Emitting Diode Array

Answer: (A)

Light from a fluorescent tube is produced by the phosphor coating on the inside of the glass glowing after being stimulated by ultraviolet light. When electricity excites the mercury vapor inside the tube, it emits ultraviolet radiation. That UV light is absorbed by the phosphor coating lining the inner surface, and the phosphor then re-emits visible light through fluorescence. So the visible light you see comes from that coated surface, not from heating a filament or from the mercury vapor itself. The other possibilities don’t match how fluorescent tubes generate light: heating a filament is how incandescent bulbs produce light, and an external LED array isn’t part of how a fluorescent tube emits light.