In the refraction, the wavelength gets shorter if it enters denser medium. Why is that happening? Is it just a phenomenon?
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The wavelength of light changes as light changes medium because light changes speed when it changes medium. When light passes through matter, it interacts with the electrons in the atoms of the medium. Since light is an electromagnetic wave, it affects the motion of the electrons, and the electrons in turn affect the electromagnetic field, changing the electromagnetic wave as the wave passes. A more detailed description of this effect can be found here: microscopic explanation of the refractive index
When the light slows down, it maintains the same frequency (since the medium has a mostly linear response). Frequency, wavelength, and speed are related as so:
$$v = f \lambda$$
Therefore, if the velocity $v$ decreases, and the frequency $f$ stays the same, the wavelength $\lambda$ must decrease.
Brionius
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