= ==: = == DISPERSION SCATTERING DISPERSION OF LIGHT: DISPERSION OF LIGHT Dispersion of light is the phenomenon of splitting of a beam of white light into its constituent colours on passing through a prism. The band of seven colours so obtained is called the (visible) spectrum. The order of colours from the lower end of the spectrum is Violet (V), Indigo (I). DISPERSION OF LIGHT: DISPERSION OF LIGHT Cause of dispersion . Each colour has its own characteristic wavelength ( ). According to Cauchy’s formula, refractive index ( ) of a material depends on wavelength ( ) of light as …(7) where A, B, C are constants. DISPERSION OF LIGHT: DISPERSION OF LIGHT of material of prism is different for different colours . For prisms of small refracting angle, we have shown that angle of deviation is As of prism is different for different colours , therefore, different colours deviate through different angles on passing through the prism. ANGULAR DISPERSION : ANGULAR DISPERSION Angular dispersion produced by a prism for white light is the difference in the angles of deviation of two extreme colours i.e., violet and red colours . DISPERSIVE POWER: DISPERSIVE POWER The dispersive power of a prism is defined as the ratio of angular dispersion to the mean deviation produced by the prism. It is represented by The mean deviation produced by the prism is As dispersive power = SCATTERING OF LIGHT: SCATTERING OF LIGHT Most of the beautiful phenomena like Blue colour of sky, white colour of clouds, have been explained in terms of scattering of light. SCATTERING OF LIGHT: SCATTERING OF LIGHT As sunlight travels through the earth’s atmosphere, it gets scattered by the large number of molecules present in the atmosphere. Scattering represents basically change in the direction of light . SCATTERING: SCATTERING Scattering of light occurs when a beam of light is broken into several smaller, less intense beams of light by gases, aerosols, or particulates in the atmosphere. TYPES OF SCATTERING: TYPES OF SCATTERING LORD RAYLEIGH was the first to deal with scattering of light from air molecules. As intensity of scattered light (I s ) varies directly as square of amplitude of scattered light (a 2 ), therefore, i.e. intensity of scattered light varies inversely as the fourth power of the wavelength of incident light. TYPES OF SCATTERING: TYPES OF SCATTERING RAYLIEGH SCATTERING: RAYLIEGH SCATTERING Another important factor in scattering is relative size of the wavelength of light ( ) and the size of scatterer ( of typical size, say, x). For x < < , Rayleigh scattering is valid, i.e.,when size of scatterer is much less than wavelength of light, . SCATTERING: SCATTERING Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. RAMAN SCATTERING Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering . Usually this kind of scattering involves exciting some vibrational mode of the molecules, giving a lower scattered photon energy, or scattering off an excited vibrational state of a molecule which adds its vibrational energy to the incident photon.