题目内容
用透明物质做成内、外半径分别为a,b的空心球的内表面上,涂有能完全吸光的物质,当一束平行光射向此球时,被吸收掉的光束的横截面积
,如图所示。不考虑透明物质的吸收和外表面的反射,试求该透明物质的折射率n 。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250051512674865.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151251552.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250051512674865.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151283344.png)
试题分析:因临界光线是折射后与a球面相切的光线,根据题设做出光路图如图所示
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250051513143117.jpg)
由几何关系得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151329673.png)
根据折射定律可得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151345740.png)
两式联立解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151563716.png)
设被吸收掉的光束的半径为R,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151579581.png)
又被吸收掉的光束横截面积为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151595684.png)
联立解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825005151626472.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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