tube light

Hi,
here iam finding difficulty to know the reason behind blackening of tubes ....
what is the reason that when a tube is switched on the ends will glow orange in colour
and the middle part will be black.... i know that this indicates that the life of
the tube got over. Please let me know the reason behind this process...

Give this link a read with a focus on principles of operation. Over years of use the filaments (heaters) produce the dark spotting (black areas). That should help.

Ron