The decreasing order of wavelength of infrared, microwave, ultraviolet and gamma rays is

The decreasing order of the wavelength for the given rays is opposite to the order of their frequencies because the frequency of the wave is inversely proportional to the wavelength.

$\mathrm{i}.\mathrm{e}.\mathit{ }\nu \propto \frac{1}{\mathrm{\lambda }}$

The order of frequency for the waves is microwave < infrared < UV < gamma rays.

Therefore, the decreasing order of wavelength is: 
microwave > infrared > UV >  gamma rays