When light passes from one medium to another, which property remains **unchanged**?
AWavelength
BSpeed
CFrequency
DAmplitude
Answer & Solution
Correct answer: C. Frequency
**Frequency** is set by the source and doesn't change at a boundary (otherwise oscillating dipoles at the interface would be discontinuous). Wavelength and speed both change in proportion: $\lambda_2 = \lambda_1 \, n_1/n_2$, $v = c/n$.
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