Wave travelling from one medium to another

Iqbal Lucky asked

"when a wave enters from air to water, its wavelength changes but frequency remains unchanged. why"

Answer Velocity of a wave = frequency x time period V = ν λ When a wave travels from one medium to another, its velocity changes and so also is its wavelength, but frequency; which is the number of vibrations produced per second is independent of the medium, remains constant. However, their is a phenomenon called Doppler Effect where the apparent frequency (observed frequency) is different from actual frequency and is caused by the relative movement between the source and observer (listener) Got a better answer? Please post it as comment.