IS THERE ANY DERIVATION ON WHY THE GRAPH OF ANGLE OF INCIDENCE AND THE ANGLE OF DEVIATION FIRST DECREASES AND THE INCREASES

Dear student 

This means that if you move along a particular direction, then the angle of emergence first decreases (on rotating towards the angle of minimum deviation position), then reaches a minimum value (equal to the angle of incidence) at the minimum deviation position and after that it again starts to increase

Hope his helps 

Good Luck