Optical spectroscopic methods of analysis are based on interaction of light with matter. Spectra arise due to energy excitations at electronic, atomic or molecular levels.
Light travels at a uniform speed in a straight direction till it encounters a medium of different optical properties. The present article discusses interactions when a light beam encounters another such medium.
Absorption and Transmission
The degree of absorption depends on the nature of absorbing medium. A fully opaque object absorbs all the incident radiation and as it neither reflects nor transmits light so it appears black. On the other hand a transparent medium transmits all the incident light falling normally upon it and appears to be colourless. A translucent body transmits only partially and appears opaque. The degree of opacity depends upon the extent of light transmitted.
It is rare to come across perfect transmitters or absorbers. Generally materials are partial transmitters. Specific wavelengths are absorbed and the colour of the media is due to complimentary wavelengths that are transmitted or reflected
Reflection takes place when a light beam reaches a polished smooth surface. The angle of reflection is equal to the angle of incidence. In ideal case the entire incident beam gets reflected.
A light beam passing from a medium to another having different density undergoes a change in direction provided it is not aligned normal to the surface at the point of entry. The angle of refraction is less than the angle of incidence if the second medium is denser but greater than the angle of incidence if the second medium is rarer. Refractive index of a medium is a characteristic property and is useful to define the purity of a material
Scattering or Dispersion
Scattering or dispersion is the reflection of light resulting from uneven surface. The intensity and direction of scattered light is dependent upon the angle of the reflecting surface point with reference to the incident beam. Light scattering also depends on particle size. Scattering due to particle smaller in size than the incident light wavelength is called Rayleigh scattering
Light comprises of electromagnetic waves in which the vibrations are present in infinite number of planes perpendicular to the direction of propagation. On passing through a polarizer vibrations in all other planes are blocked except in one particular plane which are allowed to pass through. The phenomenon is referred to as plane polarization of light.
Polarized light is applied mainly for determination of specific optical rotation of optically active molecules and identification of optically active isomers.
It is essential for a spectroscopist to acquire knowledge on such behavior of light in order to gain a better insight into different spectroscopic techniques.