lengthen vs prolong

lengthen and prolong both are verbs.

Word	Noun	Adjective	Verb	Adverb
lengthen	No	No	Yes	No
prolong	No	No	Yes	No

As verbs, prolong is a hyponym of lengthen; that is, prolong is a word with a more specific, narrower meaning than lengthen:

Other hyponyms of lengthen include draw out, extend, protract, elongate, stretch.

lengthen (verb)	prolong (verb)
become long or longer	lengthen in time; cause to be or last longer
make longer	lengthen or extend in duration or space