# How does distance affect light intensity? [closed]

Why does light intensity decrease as the distance increases from the light source?

## closed as off-topic by stafusa, JMac, Jon Custer, sammy gerbil, JamalSOct 24 '17 at 16:00

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Because of conservation of energy. Say you envelope a small light bulb with an imaginary sphere such that the bulb is at its centre. The energy that passes through your sphere in a unit of time is: $$\frac{\Delta E}{\Delta t} = I\cdot A$$ where $A$ is the area of the sphere. Now if you pick a larger sphere then the energy flux must be the same. I.e $\Delta E/ \Delta t$ is the same. But $A$ gets bigger, therefore $I$ must get smaller. As $A \propto d^2$, $I$ gets smaller as $d$ gets larger.