The figure (6) shows fraunhoffer's diffraction due to a single slit. It first minimum is obtained in the direction shown, then the path difference between rays 1 and 3 is

The condition for minimum intensity in case of single slit is given byesin⁡θ=±mλThis equation gives the directions of first, second, third,... minima by putting m = 1,2,3,...,etc. For first minima(m= 1)dsin⁡θ=λ Path difference between rays 1 and 3rd is λ.