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Light is incident normally on a glass slab of refractive index 1.5. If 4% of the light is reflected and the incident light's electric-field amplitude is 30 V/m, what is the electric-field amplitude of the wave that travels into the glass?

1. 24 V/m
2. 30 V/m
3. 10 V/m
4. 6 V/m

Correct answer: 24 V/m

Solution

Intensity is proportional to amplitude squared. The reflected fraction is 4%, so the reflected amplitude is sqrt(0.04) = 0.2 of incident. By the boundary continuity of the tangential field (E_incident = E_reflected + E_transmitted for normal incidence), the transmitted amplitude follows.

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