from a surface is completely polarized if the refiected beam and the beam refracted into the material form a right angle. The situation is illustrated in Fig.33-4.If the incident beam is polarized in the plane of incidence, there will be no reflection at all. Only if the incident beam is polarized normal to the plane of incidence will it be reflected. The reason is very easy to understand. In the reflecting material the light is polarized transversely, and we know that it is the motion of the charges in the material which generates the emergent beam, which we call the refiected beam. The source of this so-called reflected light is not simply that the incident beam is reflected; our deeper understanding of this phenomenon tells us that the incident beam drives an oscillation of the charges in the material, which in turn generates the reflected beam. From Fig 33-4 it is clear that only oscillations normal to the paper can radiate in the direction of reflection, and consequently the refle beam will be polarized normal to the plane of incidence. If the incident beam is polarized in the plane of incidence, there will be no reflected light. This phenomenon is readily demonstrated by reflecting a linearly polarized beam from a flat piece of glass. If the glass is turned to present different angles of Fig. 334. Reflection of linearly po. incidence to the polarized beam, sharp attenuation of the reflected intensity is arized light at ' s angle. The observed when the angle of incidence passes through Brewster's angle.This indicated by attenuation is observed only if the plane of polarization lies in the plane of incidence dashed dots indicate If the plane of polarization is normal to the plane of incidence, the usual reflected polarization normal to the paper. intensity is observed at all angles 33-5 Optical activity Another most remarkable effect of polarization is observed in materials composed of molecules which do not have reflection symmetry: molecules shaped something like a corkscrew, or like a gloved hand, or any shape which, if viewed y through a mirror, would be reversed in the same way that a left-hand glove reflects 4& e, as a right-hand glove. Suppose all of the molecules in the substance are the same, . e,, none is a mirror image of any other. Such a substance may show an interesting effect called optical activity, whereby as linearly polarized light passes through the substance, the direction of polarization rotates about the beam axis To understand the phenomenon of optical activity requires some calculation, ig. 33-5. A molecule with a shape but we can see qualitatively how the effect might come about, without actually mirror. Abeam of light, linearly polarized carrying out the calculations. Consider an asymmetric molecule in the shape of the y-direction, falls on the molecule. a spiral, as shown in Fig. 33-5. Molecules need not actually be shaped like a corkscrew in order to exhibit optical activity, but this is a simple shape which we shall take as a typical example of those that do not have reflection symmetry When a light beam linearly polarized along the y-direction falls on this molecule the electric field will drive charges up and down the helix, thereby generating a current in the y-direction and radiating an electric field Ey polarized in the y-direc- tion. However, if the electrons are constrained to move along the spiral, they must also move in the x-direction as they are driven up and down. when a current is flowing up the spiral, it is also flowing into the paper at z= z1 and out of th paper at z=21+ A, if A is the diameter of our molecular spiral. One might ppose that the current in the x-direction would produce no net radiation, since the currents are in opposite directions on opposite sides of the spiral. However, if we consider the x-components of the electric field arriving at z= z2, we see that the field radiated by the current at z= z1+ A and the field radiated from z= Z1 arrive at z2 separated in time by the amount A/c, and thus separated in hase by t w d/c. Since the phase difference is not exactly T, the two fields do not cancel exactly, and we are left with a small x-component in the electric field generated by the motion of the electrons in the molecule, whereas the driving electric field had only a y-component. This small x-component, added to the large ponent, produces a resultant field that is tilted slightly with respect to the the original direction of polarization. As the light moves through the aterial, the direction of polarization rotates about the beam axis. by drawing a few examples and considering the currents that will be set in motion by an incident 33-6