if the distance between a point soutce of light and a surfac

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if the distance between a point soutce of light and a surface is tripled,the intensity if
if the distance between a point soutce of light and a surface is tripled,the intensity of illumination on the surface will be
a,tripled
b,doubled
c,reduce to1/2
d,reduce to1/3
e,reduce to1/9
想问一下,这题有公式吗?为什么选E

平方反比定律 (照度第一定律）:
E = I/r^2
E为照度；I 为发光强度；r为光源至被照射物体表面的距离.

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