Electromagnetic (EM) Wave Response to a Row of Dipoles

This animation will visualize how a EM wave amplitude, E, is affected (or delayed) by a row of dipoles. A dipole moment is equal to e*dy where e is the electron charge and dy is the displacement of the negative charge with respect to the positive charge. It is standard practice to compute the macroscopic effects of a volume density of dipoles in a transparent medium without trying to visualize what is happening in the nanoscopic world of the individual dipoles. The macroscopic results for the delay at frequencies far below any resonance frequency are really pretty simple: the dielectric constant is epsilon=epsilon0+N*e*e/k where N is the dipoles per unit volume, e is the electron charge, and k is the dipole restoring force spring constant. Then, of course, the refractive index, n, is just n=sqrt(epsilon/epsilon0) and the refractive index represents the relative increase in spatial frequency of the wave. So then one might say that the speed of light in the transparent medium is c/n were c is the light speed in vacuum. In thid animation the dipole is represented a red dot if the inducing E field is of positive sign and a black dot if the inducing E field is of negative sign. To the left and right of the row of dipoles is a vacuum where there are no real dipoles. So the wavelength of the vacuum wave impinging on the dipole row is the index n times the wavelength inside the row. As the wave progresses through the dipole row note that the signd of the dipoles progress with it. As always, the learner is welcome to hit F12 in Windows to view the code used for this visualization.