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# PPT13 - Its Just Refraction When a beam of light encounters...

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It’s Just Refraction When a beam of light encounters a plane boundary between two media, it reflects as well as refracts. If the second medium is particularly “transparent” most of the beam will be refracted except under a special condition, to be discussed later. When refracted, the beam will not continue as if the boundary was absent; instead it will bend away from the original path . It turns down from the intended direction if from thin to thick (and up otherwise). From thin to thick , the bending occurs because the lower edge of the light wave is slowed by the medium, the upper edge overtakes it. Plane boundary - penetrable (seen from the side and with thin on top, as incident medium ) Thin Thick Reflected Refracted Intended direction

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Mr. Snell’s Law But how much down or how much up depends upon the relative speeds of light in the two media and is determined by Snell’s Law. n i The Law involves the refractive indexes of the two media and is given by : sin i / sin r = n r / n i or n i sin i = n r sin r Obeying the law : if from thin to thick , n i is less than n r . Therefore, sin i is greater than sin r and for the cases here this means the angle i is larger than the angle r. The ray turns down . Remember this is relative . The argument is spun around when the ray goes from thick to thin when the turn is up . i r Incident medium Incident angle n r Refracted angle Refractive medium n r r
Understanding the Law Objects in air viewed from inside water. Thus, if you are immersed in a swimming pool and looking up at a balloon tied at poolside, it will appear to be higher than it really is. Light from the balloon is turned down (to a smaller refraction angle) and your eyes will interpret that the beam of light from the balloon appears to be coming from a spot that is higher than it really is. The phenomenon is reversed when you view an object from the air at an object immersed in water. water Real height Apparent height air

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Understanding the Law II Swimming pools look shallower than they are. Light from an object at the bottom when it exits is turned to a larger angle r and therefore appears to come from a shallower spot.
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