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5 x 10^7 kg of radioactive material emits energy in the form of photons of red light (700 nm). (Note: photons have no mass.) What is the momentum of each photon?

We just started a new chapter, and in the process of learning it, I came across this question in the back of our book, which I am really having trouble understanding right now. Can anyone suggest how I can go about solving this (note, I would like to actually work out the problem myself; however, I need a sufficiant place to start.) Obviously, the Newtonian equations like p =m/v will not apply here, so I'm somewhat stuck at the moment...

Any help is much appreciated, thank you.

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The momentum of a photon is inversely proportional to its wavelength $\lambda$:

$p=\frac{h}{\lambda},$

where $h$ is the Planck constant.

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