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Just something I can't wrap my mind around. If light is oscillating EM field, and the strength of EM field weakens proportionally to distance squared, then why light doesn't lose energy as it travels further from the source? Why wouldn't oscillating EM field weaken over distance same as non-oscillating field does?

P.S. I've found sources saying that EM radiation is a wave that "detaches" from the source. If it's the case, could you explain why exactly would detachment allow the wave to "ignore" the weakening over distance?

Thanks.

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    $\begingroup$ If I live 100 miles from you and turn on my desk lamp, which of us do you think will receive more light? $\endgroup$
    – WillO
    Commented Jul 1, 2020 at 11:45

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As the light moves forward, it spreads out over a larger area. The total energy is the same, but a smaller fraction of the total passes through a rectangle of a given size.

My old high school physics teacher had a good mental image for this. Instead of a light source, imaging an automatic toast buttering machine at a restaurant. It melts a pat of butter and sprays it on the toast. Initially, the restaurant puts the toast at $r = 1$. For $1$ pat, $1$ slice of toast.

To save money, they moved out to $r = 2$. Now $1$ pat, $4$ slices of toast. Each slice gets less butter.

enter image description here

Picture from ResearchGate

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  • $\begingroup$ Am I correct to understand that static electromagnetic force weakens for the same exact reason (it doesn't really weaken, but is spread over larger area)? $\endgroup$
    – A.V. Arno
    Commented Jul 1, 2020 at 12:56
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    $\begingroup$ Right. Sometimes the details are different, but the idea is the same. For example, if you think about light in all directions from a light bulb, you might use an expanding sphere instead of expanding squares. A wire generates a magnetic field. That spreads out like a cylinder. $\endgroup$
    – mmesser314
    Commented Jul 1, 2020 at 14:06

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