Hi - I'm trying a problem on diffraction grating, and I keep on getting stuck. Maybe someone can help me figure out the last step: Three, and only three, bright fringes can be seen on either side of the central maximum when a grating is illuminated with light ( wavelegnth = 490 nm). What is the maximum number of lines/cm for the grating? Basically, what I've done is that I'm using the equation d sin(theta) = m lambda / 2 I figure that because its constructive itnerefrence for 3 fringes, m=3, and the wavelength will equal the aforementioned number. I also know that N= 1/d. So, I'm trying to figure out the value of sin(), but I'm not really sure what theta would equal to. How would I figure that out? Is my reasoning correct, or am I missing something critical to solving this problem? Thanks so much.