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When monochromatic light passes from vacuum into a medium of refractive index $\mu$:
ABoth frequency and wavelength decrease
BFrequency decreases, wavelength stays the same
CFrequency stays the same, wavelength becomes $\lambda/\mu$
DBoth frequency and wavelength stay the same
Answer & Solution
Correct answer: C. Frequency stays the same, wavelength becomes $\lambda/\mu$
Frequency is set by the source and cannot change at an interface. Speed in the medium is $v = c/\mu$, so $\lambda' = v/\nu = (c/\mu)/\nu = \lambda/\mu$. The wavelength shrinks in a denser medium while colour (frequency) is preserved.
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