. "??? P ??? 2 = I 2 ( A 1 A 2 ) 2 sin 2 ?? ( l - x ) / C cos 2 ?? l / C + 1 4 ( A 1 A 2 ) 2 sin 2 ?? l / C ( 2 ) Now, if one picks a length l =n ??/2, where n is an integer and A is the wavelength, Eq. 2 can be reduced to" . . . .