Electromagnetic waves known as Fraunhofer lines (Figure . ). The absorption spectra for various materials are compared with the Fraunhofer lines in the solar spectrum, which helps in identifying elements present in the Sun’s atmosphere. Fraunhofer lines When the spectrum obtained from the Sun is examined, it consists of large number of dark lines (line absorption spectrum).
These dark lines in the solar spectrum are bands which are separated by dark spaces. This spectrum has a sharp edge at one end and fades out at the other end. Such spectra arise when the molecules are excited. Band spectrum is the characteristic of the molecule and hence the structure of the molecules can be studied using their band spectra.
Example: spectra of ammonia gas in the discharge tube etc. (b) Absorption spectra When light is allowed to pass through a medium or an absorbing substance then the spectrum obtained is known as absorption spectrum. It is the characteristic of absorbing substance. Absorption spectrum is classified into three types: (i) Continuous absorption spectrum When we pass white light through a blue glass plate, it absorbs all the colours except blue and gives continuous absorption spectrum.
Figure . line absorption spectra Cold gas Absorption spectrum (ii) Line absorption spectrum When light from the incandescent lamp is passed through cold gas (medium), the spectrum obtained through the dispersion due to prism is line absorption spectrum (Figure . ). Similarly, if the light from the carbon arc is made to pass through sodium vapour, a continuous spectrum of carbon arc with two dark lines in the yellow region are obtained.
(iii) Band absorption spectrum When white light is passed through the iodine vapour, dark bands on continuous bright background is obtained. This type of band is also obtained when white light is passed through diluted solution of blood or chlorophyll or through certain solutions of organic and inorganic compounds. Figure . Solar spectrum - Fraunhofer lines Ultraviolet X-rays Y-rays KH h d h c a b - - g f e G D Infrared and radio spectrum