What is Fluorescence? - QS Study
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Fluorescence: When an atomic or molecular system is excited into higher energy state by absorption of energy, it returns back to lower energy state in a time less than 10-5 second and the system is found to glow brightly by emitting radiation of longer wavelength.

When ultra violet light is incident on certain substances, they emit visible light.

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It may be noted that fluorescence exists as long as the fluorescing substance remain exposed to incident ultraviolet light and re-emission of light stops as soon as incident light is cut off.