Here's how it works:

* Electric Discharge: When electricity is passed through the tube, it excites the mercury atoms.

* UV Light Emission: The excited mercury atoms emit ultraviolet (UV) light.

* Phosphor Coating: The inside of the tube is coated with a phosphor material. This material absorbs the UV light and emits visible light.

So, while mercury vapor is the primary gas in a tube light, it's the phosphor coating that actually produces the visible light we see.