absorption and emission of photons
Rayleigh scattering
refraction
fluorescence
resonance fluorescence etc
these are all appear to be different but closely related[1].
Atomic excitation and de-excitation is seen in the presence of photon as shown below[2].
(2) Light Scattering
An isolated atom scatters light because the electric field of the incident light wave forces the electrons in the atom to oscillate back and forth about their equilibrium position. By the laws of electromagnetism, when a charge changes its velocity, it emits radiation. Light is emitted uniformly in all directions in the plane ⊥ to oscillation, but decreases in amplitude as the viewing angle shifts away from that plane[3]
Measaurement of brightness of light is possible by caluclating the heat like power per unit area or by measuring the increased heat in black body which is exposed to light, one can meaasure the brightness of light[4].
(The light energy is being converted to heat energy, and the amount of heat energy absorbed in a given amount of time can be related to the power absorbed, using the known heat capacity of the object. More practical devices for measuring light intensity, such as the light meters built into some cameras, are based on the conversion of light into electrical energy, but these meters have to be calibrated somehow against heat measurements. )
references:
1.http://elchem.kaist.ac.kr/vt/chem-ed/light/light-ma.htm
2.http://csep10.phys.utk.edu/astr162/lect/light/molecular.html
3.www.its.caltech.edu/~ch24/lecture2324_2004.pdf
4.http://www.vias.org/physics/bk5_01_03.html
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