Condition for secondary maxima is
Angle of diffraction θ ≈ ( n + 1/2) λ/a where n = ±1, ±2, ±3.......
Condition for diffraction minima:
Angle of diffraction θ ≈ nλ/a where n = ±1, ±2, ±3.......
where λ is wavelength of light used and a is slit width.