Difference between Rayleigh scattering and fluorescence spectroscopy How can we differentiate Rayleigh scattering from fluorescence spectrum?
 A: Essentially, fluorescence "scattering" is inelastic - transferring some energy to the fluorphore and decreasing the energy / frequency of the emitted light relative to the excitation - and Rayleigh scattering is elastic - conserving energy and thus wavelength. Rayleigh scattering is also very fast: elastic scattering of photons off medium electrons and each event happens over femtosecond time intervals. A fluorescence event typically takes six orders of magnitude longer fluorescence lifetime of Fluorescein is 4.5ns). Another way of looking at this is that Rayleigh scattering is an extension of the physical processes that give rise to a medium's refractive index because the medium's atoms are discrete, not a continuum and the interaction between light and the medium is by way of virtual states. Fluorescence on the other hand involves the transition of electrons between bound states of the fluorophore. Moreover, in the raised state, some energy can be coupled irreversibly to the vibration and rotation modes of the fluorophore, so the latter decays to a metastable state awaiting the fluorescence event. 
