"When a particle changes its medium then  if velocity decreases with decrease in wavelength, Its frequency remains Constant why the frequency remains constant? Sir i need a solid reason please help

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v = (frequency)(wavelength).  Frequency is related to the number of disturbances per second which is the same in both media, it is not related to how fast the energy, wave, travels in the medium.  So if frequency doesn't change, the wavelength must adjust with the speed and in the same direction.