When monochromatic light travels from one medium to another its wavelength changes but frequency remains the same. Explain.

Explanation

Atoms (of the second medium) oscillate with the same (incident light) frequency and in turn, emit light of the same frequency.
Alternatively,
Frequency of light may be viewed as a property of the source and not of the medium.
Alternatively,
From Huygen.s principle, we find that
`(v_(1))/(lambda_(1))=(v_(2))/(lambda_(2))=n`(Frequency)
Hence frequency remains the same.