If the solar spectrum is closely examined, it is found that it consists of large number of dark lines. These dark lines in the solar spectrum are called Fraunhofer lines. Solar spectrum is an example of line absorption spectrum.
The central core of the sun is called photosphere which is at a very high temperature of the order of 14 million kelvin. It emits continuous spectrum. The sun’s outer layer is called chromosphere. This is at a comparatively lower temperature at about 6000 K. It contains various elements in gaseous state.
When light from the central core of the sun passes through sun’s atmosphere, certain wavelengths are absorbed by the elements present in the chromosphere and the spectrum is marked by dark lines.
By comparing the absorption spectra of various substances with the Fraunhofer lines in the solar spectrum, the elements present in the sun’s atmosphere have been identified.
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.
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.
There are some substances in which the molecules are excited by the absorption of incident ultraviolet light, and they do not return immediately to their original state. The emission of light continues even after the exciting radiation is removed. This type of delayed fluorescence is called phosphorescence.