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A plane wavefront AB travelling in air strikes a flat glass surface XY, and CD is the corresponding refracted wavefront. The refractive index of glass relative to air equals:

1. BD/AC
2. AB/CD
3. BD/AD
4. AC/AD

Correct answer: BD/AC

Solution

Using Huygens' construction, while the wavelet from B travels distance BD in air, the wavelet from A travels AC in glass in the same time. Since n = c_air/c_glass = (distance in air)/(distance in glass) for the same time, n = BD/AC.

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