Fluorescence 

When a beam of light is incident on certain substances, they emit visible light or on radiations and they stop emitting light or radiations and they stop emitting light or radiation as soon as the incident light is cut off. This phenomenon is known as fluorescence.

The substance that exhibit fluorescence are known as fluorescent substances.
When a substance absorbs light energy, electrons move from inner orbits to outer orbits. When these excited electrons return to the ground state, the light is emitted. This emitted light may possess a different frequency than the incident light.
X (Normal state) + hv (Photon) ⟶ X* (Excited state)
X* ⟶ X** + hv'
X** ⟶ X + hv"
Where X** represents an energy state between X and X*. The emitted radiation frequencies v' And v" are different from v, the frequency of incident radiation.

Some Characteristics of fluorescence:

(i) This phenomenon is instantaneous and starts immediately after the absorption of light and stops as soon as the incident light is cut off fluorescence is caused by the visible or ultraviolet regions of the spectrum, line, band and continuous spectra are observed.
(ii) This phenomenon is exhibited by gases, liquids and solids. fluorescence will be observed in gases if the pressure is low.
(iii) Different substances fluorescence with the light of different wavelengths. Fluorspar fluoresces with blue light, chlorophyll with red light, uranium glass with green light.
(iv) The fluorescent light from solutions is polarized and the degree of polarization depends upon the concentration of the solution.
(v) The extent of fluorescence depends upon the nature of the solvent and the presence of certain anions in the solution.
(vi) According to stoke's law, during fluorescence light is absorbed in a certain wavelength and should be emitted at a greater wavelength.
(vii) The quantum yield in fluorescence is the ratio of the number of photons of luminescent radiation to the number of photons absorbed. The quantum efficiency increases in proportional to the wavelength λ of absorbed radiation. After reaching the maximum value λmax, the efficiency drops rapidly to zero upon a further increase in λ.
(viii) Fluorescence is a secondary effect. The primary process is the absorption of a quantum of light.