How does a tube light on its both side change its colour to black.?

how is the dark colour seen in the tube light whe we use it long time? to say oxidation it is impossible because there won't be oxygen inside . so what's the ansHow does a tube light on its both side change its colour to black.?
The basic principle behind a florescent lamp is the collision of an incident electron with an atom of the gas (usually a combination of mercury and argon) present within the tube. This collision separates an ultraviolet photon from the atom - the light from which is invisible to the human eye.



The glass of these lamps are coated with a phosphor layer that absorbs the ultraviolet photon and, in turn, releases a weaker photon -- which creates the light.



A lamp goes dark at the edges primarily because of erosion of this phosphor coating -- an erosion which occurs due to chemical reactions caused by the proximity of the coated layer to the cathode and anode of the lamp.



Hope this helps.How does a tube light on its both side change its colour to black.?
Apart from other too technical matters, practically during low - line voltage, the fluoresent tube will keep on flickering/slowly picking brightness, causes the black spots on either ends. This includes CFL too.