Raman Effect - PDF

***Raman effect*** In 1928, Sir C.V. Raman discovered experimentally, that the monochromatic light is scattered when it is allowed to pass through a substance. The scattered light contains some additional frequencies other than that of incident frequency. This is known as Raman Effect. The lines w...

***Raman effect*** In 1928, Sir C.V. Raman discovered experimentally, that the monochromatic light is scattered when it is allowed to pass through a substance. The scattered light contains some additional frequencies other than that of incident frequency. This is known as Raman Effect. The lines whose frequencies have been modified in Raman effect are called Raman lines. The lines having frequencies lower than the incident frequency are called Stoke's lines and the lines having frequencies higher than the incident frequency are called Anti-stokes lines. This series of lines in the scattering of light by the atoms and molecules is known as Raman Spectrum. The Raman Effect can be easily understood, by considering the scattering of photon of the incident light with the atoms or molecules. Let the incident light consist of photons of energy hν~o~. 1.     If a photon strikes an atom or a molecule in a liquid, part of the energy of the incident photon may be used to excite the atom of the liquid and the rest is scattered. The spectral line will have lower frequency and it is called stokes line. 2\. If a photon strikes an atom or a molecule in a liquid, which is in an excited state, the scattered photon gains energy. The spectral line will have higher frequency and it is called Anti-stoke's line. 3.     In some cases, when a light photon strikes atoms or molecules, photons may be scattered elastically. Then the photons neither gain nor lose energy. The spectral line will have unmodified frequency. https://img.brainkart.com/extra/e5tYiY2.jpg ***Applications of Raman Spectrum** * 1.     It is widely used in almost all branches of science.  2.     Raman Spectra of different substances enable to classify them according to their molecular structure.  3.     In industry, Raman Spectroscopy is being applied to study the properties of materials. 4.     It is used to analyze the chemical constitution. SCATTERING OF LIGHT ------------------- When sunlight enters the Earth's atmosphere, the atoms and molecules of different gases present in the atmosphere refract the light in all possible directions. This is called as 'Scattering of light'. In this phenomenon, the beam of light is redirected in all directions when it interacts with a particle of medium. The interacting particle of the medium is called as 'scatterer'.  **Types of scattering** When a beam of light, interacts with a constituent particle of the medium, it undergoes many kinds of scattering. Based on initial and final energy of the light beam, scattering can be classified as, 1\) Elastic scattering 2) Inelastic scattering ### 1) Elastic scattering If the energy of the incident beam of light and the scattered beam of light are same, then it is called as 'elastic scattering'. ### 2) Inelastic scattering If the energy of the incident beam of light and the scattered beam of light are not same, then it is called as 'inelastic scattering'. The nature and size of the scatterer results in different types of scattering. They are                  Rayleigh scattering                  Mie scattering                  Tyndall scattering                  Raman scattering ### Rayleigh scattering The scattering of sunlight by the atoms or molecules of the gases in the earth's atmosphere is known as Rayleigh scattering. ### Rayleigh's scattering law Rayleigh's scattering law states that, "The amount of scattering of light is inversely proportional to the fourth power of its wavelength". Amount of scattering 'S' ∝ 1/λ^4^ According to this law, the shorter wavelength colours are scattered much more than the longer wavelength colours. When sunlight passes through the atmosphere, the blue colour (shorter wavelength) is scattered to a greater extent than the red colour (longer wavelength). This scattering causes the sky to appear in blue colour. At sunrise and sunset, the light rays from the Sun have to travel a larger distance in the atmosphere than at noon. Hence, most of the blue lights are scattered away and only the red light which gets least scattered reaches us. Therefore, the colour of the Sun is red at sunrise and sunset. ### Raman scattering When a parallel beam of monochromatic (single coloured ) light passes through a gas or liquid or transparent solid, a part of light rays are scattered. The scattered light contains some additional frequencies (or wavelengths) other than that of incident frequency (or wavelength). This is known as Raman scattering or Raman Effect. Raman Scattering is defined as "The interaction of light ray with the particles of pure liquids or transparent solids, which leads to a change in wavelength or frequency." The spectral lines having frequency equal to the incident ray frequency is called 'Rayleigh line' and the spectral lines which are having frequencies other than the incident ray frequency are called 'Raman lines'. The lines having frequencies lower than the incident frequency is called stokes lines and the lines having frequencies higher than the incident frequency are called Antistokes lines. You will study more about Raman Effect in higher classes.