English: Deriving the grating condition; at perpendicular incidence α+φ=90°, inside the right-angled triangle the other angle is equal to 90°-α = φ, constructive interference occurs when sin φ is equal to the wavelength divided by the distance between the rulings, or more generally when ${\displaystyle \sin(\varphi )={\dfrac {n\lambda }{d}},n\in \mathbb {Z} .}$