平方反比定律者，立圓表面積之推論也
設發射單位 I 0 {\displaystyle I_{0}} ，強度單位 L {\displaystyle L} ，立圓表面積 4 π R 2 {\displaystyle 4\pi R^{2}} ，則有
L = I 0 4 π r 2 = I 0 4 π ⋅ r − 2 {\displaystyle L={\frac {I_{0}}{4\pi r^{2}}}={\frac {I_{0}}{4\pi }}\cdot r^{-2}}
引力
F g = G ⋅ m 1 m 2 r 2 {\displaystyle F_{g}=G\cdot {\frac {m_{1}m_{2}}{r^{2}}}}
庫侖力
F q = k ⋅ q 1 q 2 r 2 {\displaystyle F_{q}=k\cdot {\frac {q_{1}q_{2}}{r^{2}}}}
普通光照
L = I 0 4 π r 2 {\displaystyle L={\frac {I_{0}}{4\pi r^{2}}}}
