How are molecules affected by ultra-violet/visible radiation?

the ultraviolet and visible spectrum of most molecules consists of a few humps rather than sharp lines.

How are molecules affected by ultra-violet/visible radiation?
You have mainly sharp lines in atoms since there are only interaction between few particles mainly protons and nearby electrons. When you have molecules you have in addition with the electronic levels more interactions intra (within) and inter (outer) molecules these interactions widens the levels of energy which appears not as a sharp boundary but like "trench".
Reply:U.V light encourage halogenation of molecules since it causes generation of free radicals from halogens.