Why does mercury become white in tube lights and how?

Mercury doesnt gives white colour. Its the phosphorent coating (probably its calcium tungstate , CaWO4) on the inside walls of the tube that gives white light. Actually the discharge in mercury vapour produces ultravoilet light. This UV when strikes the phosphorent , the phosphorent gives of white light. read more

A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light.The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. read more