# Quantum Mechanics- Antenna emitting electromagnetic radiation

Radio signals are being transmitted in a frequency of $8.4 \times 10^9 s^{-1}$ and being received by an antenne that is capable of receiveing power of $4 \times 10^{-21} watt$ ($1 watt = 1 J s^{-1}$ ) .

Estimate that number of photons per second of this electromagnetic radiation that this antenne is capable of receiving.

I can easily calculate the number of photons per second that this radio produces, dividing the emitted power by the energy of one photon,

$\hbar\omega=h\nu = h\times8.4 \times 10^9 s^{-1}$

but have no idea how to calculate the number of photons that this antenne can receive.

Can you help me?

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How can you calculate the number of produced photons if the only known thing is the frequency?

To calculate the number of photons received (or emitted) per second, you have to divide the received (emitted) power by the energy of one photon $\hbar\omega=h\nu$.

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But how does this depend on the frequency of the radio signals? I can't understand it... Can you please detail a bit? Thanks ! –  joshua Sep 17 '12 at 18:44
Frequency of the electromagnetic wave $\nu$ is the frequency of photons constituting this wave. –  Vladimir Kalitvianski Sep 18 '12 at 11:38