In interference we put n=0 for first dark , in the formula [2n+1/2 ]lamda. why we put 0 here .in the formula [2n- 1/2 ]lamda we put n=1 .why?

 

Dear student 
n = 0 points out the absolute centre . 
n= 1 points out the first fringe from centre . 
Usually we use the formula when n starts from 0 is: 
((2n+1)/2) * Lambda 
Hope this helps 
Good Luck