Fluorescence and Phosphorescence
Fluorescence Physics :
There are certain substances which, on being illuminated by high – frequency light (blue, ultraviolet), emit light of relatively low frequency.
The emission occurs so long the substance is being illuminated.
This phenomenon of emission of light is called ‘Fluorescence’ and the substances exhibiting this phenomenon are called ‘Fluorescent Substances’.
Different substances give fluorescence of different colours (i.e., different frequencies).
For example, when violet or ultraviolet light is incident on quinine sulphate, it gives fluorescence of blue colour.
Similarly, fluoricine, barium platinocynide and uranium oxide give green fluorescence.
Uses of Fluorescence :
Fluorescence has many applications in daily life.
For example, the presence of invisible ultraviolet rays can be detected by their fluorescent effect. Now – a – days, fluorescence is used in the fluorescent tubes used for light purposes.
The inner surface of the fluorescent tube giving white light is painted with magnesium tungstate and zinc – berilium silicate.
The mercury light produced by electric discharge in the tube has ultraviolet rays in large quantity.
When these rays fall on the paint of the wall, the magnesium – tungstate produces light blue fluorescence and the zinc – berilium silicate produces yellow – orange fluorescence.
The mixture of these two produces white – light fluorescence. Similarly the tube painted with cadmium borate produces pink light, whereas the tube producing green light with zinc silicate.
Phosphorescence Physics :
Fluorescent materials emit light only so long as light is incident on them.
There are certain substances which continue emitting light for some time after the light incident on them is stopped.
This phenomenon is called ‘Phosphorescence’, e.g., if we make blue light incident on a zinc – sulphide (ZnS) screen, then it produces phosphorescence for some time.
Time of phosphorescence is different for different materials.
Uses of Phosphorescence :
Phosphorescent substances are painted on watch hands, electric switch – boards and sign – boards.
These substances absorb sunlight during day – time and illuminate during the dark night by phosphorescence.
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