题干

如图所示梯形玻璃砖ABCD,∠A=45°,∠B=105°,AB边长为L.一束平行于AB边的光线由AD边入射,经折射后在AB边中点发生全反射,直接射到BC边,且与BC边成45°角射出玻璃砖.求:

①玻璃砖的折射率;

②光线在玻璃砖内传播的时间.

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答案(点此获取答案解析)

解:①光路如图所示

由几何关系有 α+β=45°,105°+β+90°﹣α=180°

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阅读理解

B

    MSG (monosodium glutamate) is a food additive (食品添加剂), which has been widely used by many food factories all over the world since it appeared first in Japan in 1909.

    MSG is a substance that is added to some food to improve its original (原先的) taste and colour. It is used in foods around the world such as cakes, bread, frozen meats, fish and vegetables. In the beginning, people thought MSG was safe just like salt and sugar. However, in 1968 some people reported they seemed to have bad reactions (反应) to MSG after having had a meal at a restaurant. They included headaches and quick heartbeat (心跳). It was also reported that MSG excited a person's brain and caused his death in Japan. Scientists think that MSG may lead to many neurological disorders (神经紊乱). In mice studies done by a university, the evidence showed the mice didn't want to eat the food including MSG.

    Today many food companies “hide” MSG on food labels (标签) by using different names. If you see “natural flavorings”, “hydrolyzed protein” and “spices” in a food label, you are probably seeing a hidden way to “report” MSG in a food label. Every time we have a meal in a restaurant, we always feel the food very delicious to taste, but sometimes it is dangerous for our health because some poisonous food additives have been added to the food.

    To protect people's health, our governments, from central to local, are taking actions to stop factories producing poisonous food additives.