- #1

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- 160

- Homework Statement
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- Relevant Equations
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I was asked to derive the relation $$p = u/3$$ for photon gas. Now, i have used classical mechanics and symmetry considerations, but the book has solved it in a interisting way:

I can follow the whole solution given, the only problem is the one about the probability to colide the sphere!. Where does the ##cos(\theta)##, in ##dA cos(\theta)/(4 \pi r^2)## comes from? Why the top part of the sphere is benefited?

I can follow the whole solution given, the only problem is the one about the probability to colide the sphere!. Where does the ##cos(\theta)##, in ##dA cos(\theta)/(4 \pi r^2)## comes from? Why the top part of the sphere is benefited?