其减弱的比率，与我们在上文中所阐述过的地面光照随距离减弱的规律是一样的，也就是说，与距离的平方成正比。
The ratio in which this light decreases, is precisely the same as that of the terrestrial light illustrated above, viz., according to the square of the distance.
阴影深度取决于光照方向和强度，以及物体和投影面的距离。
The qualities of a shadow depend on the light direction and intensity, as well as the distance between the object and the surface where the shadow is cast.
C街道距光源的距离是A街的3倍，那么光线的昏暗程度将是A街的9倍，因为3X3 = 9，也就是说C街的光照强度只等于A街的1/9，如果这里要看清书上的字，字体就要扩大九倍。
" C " street is three times as far from the light as "A" street; hence it will be nine times darker there, for 3X3 = 9.
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