Why do radical reactions usually require UV light

Care although we give different names to these photons, in reality there exists a continuum of wavelengths from 10-11 m (for y-rays) through to hundreds of metres (for radio waves). 

The titles we use, such as microwave or X-ray are just historical artifacts, and merely describe the way scientists first encountered them. 

UV Excitation of inner and valence electrons UV-visible spectroscopy 

Visible Excitation of valence and outer-shell electrons UV-visible spectroscopy 

Infrared Excitation between vibrational quantum states Infrared spectroscopy 

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