法 is just law. It can be the Nature Law or laws under a constitution. 法律 means only the legal laws. 律 originally means rules or patterns of music, and later it expanded its meaning to general rules. The major difference is that 律 is much less official than 法 when we are talking about legal issues. However, neither 法 nor 律 appears in daily usage. We just use 法律 as a single word. Hope that helps!

法：means rule made by the lord or emperor.from ancient time to modern time，the mean of this word barely changed. 法律：same as 法，the two characters of this word： “法”“律” exactly have the same meaning，this word just came out because the western culture‘s influence，some Chinese people want to get some revolutionary change of ancient Chinese language，so they made many “two characters words” to replace the “one character words”.for example:“工作”（作），“法律”（法），“回答”（答），“不同”（异），“相同”（同）。 律：At beginning，律 have the same meaning as 法，they can replace one to another.as the time pass by,chinese call the Codified law:律，such as “大清律”（the law of Qing dynasty）“大明律”（the law of Ming dynasty），because the law means rule that can not change，the law should obey，so，律got another meaning“music pattern”