Imagine light of frequency [itex]\Omega[/itex] enters a liquid and propagates in the z direction. Its velocity is reduced to c/n. This leads to reduction of its wavelength to 1/n of the vacuum wavelength and the wave is described by the macroscopic electric field [tex] \mathbf E (\mathbf x,t) = \mathbf E_0 \cos(\Omega t - n\Omega/c z). [/tex] However, does the reduction of the wavelength and speed occur also at the microscopic level? Take one molecule of the liquid; it is surrounded by empty space, and is under action of the microscopic electromagnetic field. How would you describe this microscopic field, as a slowed-down wave [tex] \mathbf e (\mathbf x,t) = \mathbf e_0 \cos(\Omega t - n\Omega/c z). [/tex] or as a vacuum wave [tex] \mathbf e (\mathbf x,t) = \mathbf e_0 \cos(\Omega t - \Omega/c z). [/tex] or neither?