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Moved from a technical forum, so homework template missing

Hi people, I studying electromagnetic waves (intermediate) and

I don't understand how the expression for linear momentum of a wave is obtained, if the wave doesn't carry any mass.

In particular, I have to explain why the radiation pressure on a perfect absorber is half that on a perfect reflector

So, I do this:

P_rad= pressure

\vec{p}= momentum of wave

A= transversal area

Volumen= ctA

p_den= density of momentum =p/vol

but i the books're saying that E/c =p ???? Why?

Finally, p_den *c = I *c/ c² = I/c to case of absorbent surface.

I don't understand how the expression for linear momentum of a wave is obtained, if the wave doesn't carry any mass.

In particular, I have to explain why the radiation pressure on a perfect absorber is half that on a perfect reflector

So, I do this:

P_rad= pressure

\vec{p}= momentum of wave

A= transversal area

Volumen= ctA

p_den= density of momentum =p/vol

^{ P_rad=\frac{Δp}{Δt *A} =\frac{ p_den *(c *t*A)}{ t*A} = p_den *c }but i the books're saying that E/c =p ???? Why?

Finally, p_den *c = I *c/ c² = I/c to case of absorbent surface.

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