问题 选择题

一个半径为5m的圆形蓄水池装满水,水面与地面相平,在池的中心上空离水面3m处吊着一盏灯,一个身高1.8m的人离水池边缘多远的距离恰能看到灯在水中的像(  )

A.1.8m

B.2.5m

C.3m

D.5m

答案

答案:C

水池面相当于平面镜,作反射光光路图如图所示.若人在某位置时恰能通过水池的边缘看到灯在水中的像,这时光源发出的光恰好通过水池边缘反射入人的眼睛.

解:由反射定律可知,由几何分析得:△SBO∽△DCO,故有h/H=x/r

故x=hr/H=1.8×5/3=3 m

单项选择题
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