What is the difference between khalaqa and ja`ala in the qur'an?

in sura al-Anaam allah uses "khalaqa" خَلَقَ for samawat (heavens) and ard (earth) but "ja´ala" جَعَلَ for zulumat (darkness) and noor (light): > الْحَمْدُ للَّهِ الَّذِى **خَلَقَ** السَّمَـوَتِ وَالاٌّرْضَ وَ**جَعَلَ** الظُّلُمَـتِ وَالنُّورَ ثْمَّ الَّذِينَ كَفَرُواْ بِرَبِّهِمْ يَعْدِلُونَ > > [All] praise is [due] to Allah , who **created** the heavens and the earth and **made** the darkness and the light. Then those who disbelieve equate [others] with their Lord. (6:1 ) What is the difference between the two words and why did Allah uses the different words. jzk nabiha