 Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2011/paper-1/section-a/a4/solution

R = (\frac{n - 1}{n + 1})^{2} = 0.04.

The two air–glass interfaces each transmit a fraction

1 - R

of their incident power. Omitting all internally reflected paths,

\frac{I _{transmitted}}{I _{incident}} = (1 - 0.04)^{2} = 0.9216, reduction = 7.84% ≃ 8%.

The power-transmission factors, rather than electric-field amplitudes alone, are multiplied. Absorption, coatings and interference are excluded by this approximation.

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