1. The problem statement, all variables and given/known data Assume that the limits of the visible spectrum are arbitrarily chosen as 430 and 680 nm. Calculate the number of rulings per millimeter of a grating that will spread the first-order spectrum through an angle of 20.0 degrees. 2. Relevant equations (y is lambda) dsinø = my (maxima) 3. The attempt at a solution So, the first-order spectrum of white light has a line (maxima) the lowest wavelength and another line at the highest wavelength - right? And for the first-order spectrum would have a m of 1, right? If this is true, then, dsin(ø1) = 430 dsin(ø2) = 680 And, ø2 - ø1 = 20 degrees And ø1 = arcsin(430/d) ø2 = arcsin(680/d) But, I want to find out d. How can I extract d from these equations? Or, alternatively, do I not understand what's going on?