POLARIZATION BY REFLECTION
Unpolarized light can be polarized, either partially or totally, by reflection. In Fig. 34-20, unpolarized natural light is incident on a reflecting surface between two transparent optical materials; the plane containing the incident and reflected rays and the normal to the surface is called the plane of incidence. For most angles of incidence, waves for which the electric-field vector E is perpendicular to the plane of incidence (that is, parallel to the reflecting surface) are reflected more strongly than those for which E lies in this plane. In this case the reflected light is partially polarized in the direction perpendicular to the plane of incidence. E lies in the plane of incidence is not reflected at all but is completely refracted. same angle of incidence the light for which E is perpendicular to the plane of a::::dcI1lCe is partially reflected and partially refracted. The reflected light is therefore rely polarized perpendicular to the plane of incidence, as shown in Fig. 34-20. refracted light is panially polarized parallel to this plane; the refracted light is a of the component parallel to the plane of incidence, all of which is refracted, and remainder of the perpendicular component. In 1812 the British scientist Sir David Brewster discovered that when the angle of .dence is equal to the polarizing angle 9p’ the reflected ray and the refracted ray are perpendicular to each other (Fig. 34-21). In this case the angle of refraction 9b becomes complement of 9p’ so 9b = 90· – 9p• From the law of refraction,
This relation is known as Brewster’s law. Although discovered experimentally, it can also be derived from a wave model using Maxwell’s equations. Polarization by reflection is the reason polarizing filters are widely used in sunglasses.
When sunlight is reflected from a horizontal surface, the plane of incidence is vertical, and the reflected light contains a preponderance of light that is polarized in the horizontal direction. When the reflection occurs at a smooth asphalt road surface or the surface of a lake, it causes unwanted glare. Vision can be improved by eliminating this glare. The manufacturer makes the polarizing axis of the lens material vertical, so very little of the horizontally polarized light is transmitted to the eyes. The glasses also reduce the overall intensity of the transmitted light to somewhat less than 50% of the intensity of the unpolarized incident light.