A question on an introductory text to quantum mechanics asks me to calculate the number of photons striking a $1$ metre square aerial each second at a distance of $1000km$ from a $200kW$ transmitter that broadcasts radio waves at a frequency of $200kHz$. I completely understand the solution apart from a small part where it says the aerial occupies $1$ square metre out of $4\pi (10^6)^2$. Where did the $4\pi$ come from???
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2Check out the formula for the area of a sphere. – Ethan Bolker Dec 08 '17 at 14:48
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lol thank you, i was being an idiot :( – Deke Dec 08 '17 at 14:51