Maxima in a diffraction grating

[L_landau]

Homework Statement

A diffraction grating is made up of slits of width 300 nm with separation 900 nm. The grating is illuminated by monochromatic plane waves of wavelength l 600 nm at normal incidence. How many maxima are there in the full diffraction pattern?

Homework Equations

mλ = dsinθ

The Attempt at a Solution

I thought that I could just use the above equation and doing so I get m = 1.5. However, the solution uses
(a+b)sinθ = mλ . Can I really just add the slit width and the separation width like that?

 

[mfb]

You need the distance between the centers of adjacent slits, if 900 nm is the space between their edges you have to add the slit width.

 

[L_landau]

Oh that makes sense! Thank you!