Past exam of the physics B course of the University of Cambridge / 2014 / Paper 1 / Section B / B8 / b

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- Solution b

Solution

At a perfect conductor, the reflected electric field cancels the incident tangential field, while their magnetic fields add. Thus

E_{y} (0) = 0

and

H_{z} (0)

has peak amplitude

2 A / Z_{v}

. Taking the zero-penetration limit, including its surface force, gives

P_{refl} = ϵ_{0} A^{2} = \frac{2 S _{inc}}{c} .

Reversal of the normal momentum doubles the pressure compared with absorption at the same incident Poynting flux.

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