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

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

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As we know intensity is energy flowing normally per second through unit area,so with increase in distance from the source it decreases as it is inversely proportional to square of distance if the wavefront is spherical and inversely proportional to the distance if the wavefront is plane.

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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.

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  • $\begingroup$ Thank you so much for helping me out. This is really a helpful answer. $\endgroup$ – KBTW Oct 24 '17 at 6:43

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