Notice from the geometry that tanθ ≃ θ ≃ y/L, and d sinθ ≃ d θ ≃ path difference, namely some number times λ. For example, for the first bright line above the central one, we have to have a path difference of λ, so that y = λL/d. In general, for bright lines, y is mλL/d where m is any positive or negative integer, including zero. For dark lines, y is (m + (1/2))λL/d. Instead of “remembering a formula,” it is vital always to work with the actual path difference in reasoning out the interference situation.