题目

已知人眼瞳孔的直径为d,一秒钟进入瞳孔N个波长为λ的光子时,就能引起视觉.若辐射功率为P的点光源,能发出波长为λ的光,则此光源能被人眼看到的最大距离是___________. 答案：解析:点光源一秒钟辐射的光子数为n=,欲能引起视觉,有N=×πd2,R=.   The last question in the maths test yesterday was far _____ me．I couldn’t work it out _____my own．A．over , byB．above, by C．from, onD．beyond, on