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| 1.2 |
Optical response of matter |
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| In an elementary optics course, we learn that light is electromagnetic in nature. The main cause of its interaction with matter has to lie in the interaction between the electric and magnetic fields and the charged particles in matter. For transparent dielectrics, a useful model is that the oscillating electric field of light sets up electric dipole oscillations in the molecules or atoms of the medium, and they in turn radiate electromagnetic waves. In the first approximation, we expect that the induced moments would be proportional to the incident fields and would therefore oscillate at the same frequency. This field radiated by the dipoles together with the incident field produces the resultant optical waves. When the medium is homogeneous and the interface smooth then these fields form the reflected and the refracted waves. When there are fluctuations either in the medium or at the interface, there is scattering. If the optical wave has a frequency such that the corresponding photon energy matches with a possible transition energy in the medium, we learn that the medium can absorb energy from the optical beam - which may be partly re-emitted by the medium (luminescence) and party converted into other excitations of the medium which in a very broad way is called heating of the medium. |
All these phenomena, are covered by what we will now call linear optics, since the amplitude of the induced dipole is taken as proportional to the incident field it is linearly dependent on the incident field. Yet another way of saying the same thing is that the optical response of the medium is linear where we envisage that the incident light is a probe of the medium, and the resultant phenomena reflection, refraction, scattering, absorption and luminescence are all response of the medium. In the simple picture, all these responses are describe by the polarization  which is the induced dipole moment per unit volume at  at time t. |
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