 Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2014/paper-1/section-b/b8/a/solution

With no reflected field,

\overline{E_{y}^{2}} = A^{2} /2

and

\overline{H_{z}^{2}} = A^{2} / (2 Z_{v}^{2})

at the entrance. Thus

P_{abs} = \frac{1}{2} ϵ_{0} A^{2} = \frac{S _{inc}}{c}, \overline{S}_{inc} = \frac{A ^{2}}{2 Z _{v}} .

Complete absorption transfers the incident normal momentum flux to the body.

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