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lecture7 - Lecture 7 Notes 07 11 Reflection and refraction...

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Unformatted text preview: Lecture 7 Notes: 07 / 11 Reflection and refraction When an electromagnetic wave, such as light, encounters the surface of a medium, some of it is reflected off the surface, while some crosses the boundary and enters the material. The direction of travel of the wave in the material will typically change; this is called refraction . The arrows in the diagrams are known as rays , and represent the direction of travel of the wave. The rays are always perpendicular to the wave fronts. For large-scale phenomena (large-scale compared to the wavelength) involving plane waves, the electromagnetic waves all travel in the same direction and in straight lines, and rays are a useful way to represent their path. The behavior of rays under these circumstances is described by geometric optics . For phenomena that occur on a scale comparable to the wavelength, the ray approximation is not adequate, and we must keep detailed track of the actual electromagnetic waves; this is described by wave optics . The law of reflection The law of reflection states that the angle of reflection of the wave is the same as the angle of incidence: This is due to a pair of symmetries the problem possesses. Time reversal invariance is a property of electromagnetism, states that if we reverse the direction of an EM wave, it will retrace its path. The setup in the diagram is also symmetric if we flip the diagram left to right. Doing both transformations (flipping the diagram and reversing the direction of the rays) results in the two angles being exchanged. Therefore, by symmetry, the angles must be the same. Reflection and images Reflection of the rays off a smooth surface results in the creation of an image. An image is an apparent source of light rays. This is a ray diagram for an arbitrarily selected pair of rays from an object that reflect off a mirror, or another smooth reflective surface: The rays are reflected off the surface of the mirror and travel to the observer, as shown on the left side of the diagram. The observer detects these rays and traces them back to their apparent source on the right, behind the mirror. These rays appear to come from an image. Of course, there are really no rays behind the mirror, but from the observer's point of view, the situation is indistinguishable from the one where the image is the real source of the rays. Thus, the observer sees the image as if it was the actual object. An image like this, which is not a source of actual rays but rather the result of the observer's eyes tracing the rays back to their apparent origin, is called a virtual image . In other situations, we will see real images , which are formed at intersections of actual rays of light....
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This note was uploaded on 09/19/2011 for the course PHYS 1C taught by Professor Smith during the Spring '07 term at UCSD.

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lecture7 - Lecture 7 Notes 07 11 Reflection and refraction...

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