What is photo-luminiscence?

It is the phenomenon of re-radiation of absorbed energy. It is of two types : (a) Fluorescence, (b) Phosphorescence.
(a)Phosphorescence is delayed emission of long wave radiation by irradiated substances which continues for some time after removal of irradiation source.
(b)Flourescence is the property of almost immediate emission of long wave radiations by a substance after having absorbed radiation energy.