If the second polaroid is oriented the same as the first, then all the light gets through, and the intensity is unchanged, and if its polarizing axis is at right angles to the first polaroid, then no light will get through it. Clearly when the light reaches the second polaroid it will be plane-polarized from the first one. Now let's consider what happens if we send the natural light through two polaroids in succession. We can express the fact that half of natural light gets through a polaroid in a diagram as follows:įigure 3.7.2 – Natural Light Intensity Halves Through a Polaroid We look at the more general case of intensity next. By “half the light gets through,” what do we mean? We mean that the intensity drops by one half. Because the wave polarization directions are randomly-oriented, there is no reason to expect there to be a greater sum of components along one axis than another. To see why this should be so, break every electric field vector of every wave into components parallel and perpendicular to the polarizing axis. When such light is passed through a polaroid, half the light gets through. In fact “natural” light from light bulbs and the sun is “unpolarized,” which comes about because each of the individual light sources (atoms) are aligned in random orientations, and all send out random, unaligned light waves. The second oversimplification is that not all of the individual light waves that come from a source are necessarily polarized in the same direction. What happens then? Well, electric fields are vector fields, which means they can be broken into components, so the component of the electric field that is parallel to the polarizing axis gets through, and the other component is absorbed. ![]() First of all, a light wave does not have to arrive at the polarizer in either a parallel or perpendicular orientation – it could be aligned at any angle with the polarizing axis. We have overly-simplified things here, in a couple of ways. But today we like our movies to be in realistic colors, so someone came up with the idea of projecting the two images with differently-polarized light, and then give viewers glasses that only admit the properly-polarized light into the respective eyes. The original inventor did this with colors – red lenses obscure red images, and yellow lenses obscure yellow light, so films were recorded from two perspectives, and each perspective was projected in a different color – one red and one yellow. Then the trick is to make the right-perspective image invisible to the left eye and the left-perspective image invisible to the right eye, so that each eye sees only its own perspective. One image is recorded from the perspective of the right eye, and the other from the perspective of the left eye. This inventor’s idea was to project not one but two images on the same screen. Your finger’s position appears to change relative to the background. You can see this is true by holding up your finger in a fixed position and alternately opening-and-closing each eye. Your right eye sees objects from one perspective, while your left eye sees it from a slightly different perspective. The idea is based on the fact that a large component (but not the only one) of seeing in 3-D is stereo vision. Long ago someone came up with a brilliant idea for making movies projected onto a 2-D screen appear in 3-D. ![]() One interesting application of this phenomenon is 3-D movies. Such a filter is called a polaroid or polarizer. the substance is transparent to this light), while if the light is polarized perpendicular to the polarizing axis, then virtually all of the light is absorbed. When the light polarization is aligned with what we define as the polarizing axis of the substance, then little of the light is absorbed by the substance (i.e. If the light is plane-polarized (see Figure 3.1.1), then its propagation through a medium will be affected by the preferential orientation of charge oscillations. This material can have a dramatic effect on light passing through it. The upshot of this is that the charges react to electric fields along one direction (or rather, components of electric fields along one direction), while they don't react along a perpendicular direction. It so happens that it is possible to construct a solid substance which greatly restricts oscillatory motion of electric charges along a single dimension. There is little we can say about it in this class, except to say that because the light wave is electromagnetic in nature, it interacts with electric charge, which is present in all matter. \)Īs stated previously when discussing the speed of light waves through transparent media, the mechanisms that govern light propagation through media are complicated.
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