Chapter 28 - Physical Optics: Interference and Diffraction - Problems and Conceptual Exercises - Page 1008: 87

$76\mu m$

We can find the required linear distance as follows: $\Delta y=y_2-y_1$ $\implies \Delta y=\lambda_2LN-\lambda_1LN$ $\implies \Delta y=LN(\lambda_2-\lambda_1)$ We plug in the known values to obtain: $\Delta y=(225cm)(494lines/cm)[(589.59\times 10^{-9}m)-(588.99\times 10^{-9}m)]$ $\Delta y=75.582\times 10^{-6}m=76\mu m$