Recall that we are in the realm of geometrical opticswhenever the size of obstacles or apertures encountered by a propagating light wave is much greater than the wavelength  of the light. If, on the other hand, the size of obstacles or apertures is about the same size or smaller than the wavelength of the light, then we must acknowledge that the light is really an oscillating electromagnetic wave (EM wave) and work within the realm of wave optics.  So far in this course we have satisfied the conditions for geometrical optics, so that we were justified in treating light as a ray.  We will now start our discussion of light when observed in such a way that its wave properties manifest themselves. Both this lecture and the next will discuss effects in which the wave nature of light plays a dominant role in explaining the observed phenomena. This lecture will deal with the subject of polarization.  The  next lecture will deal with diffraction effects.