# Optics : Diffraction gratings.

1. I am attempting a question from a textbook but the wording or perhaps the question itself is confusing me.
*Light falls at perpendicular incidence on a transmission diffraction grating. The second order diffracted light leaving the grating is examined.
The grating has 600 slits per mm, a total width of 10 cm, and is being used to examine spectral features near a wavelength of 450 nm. How close ( in nm) can the wavelength of two spectral lines be, for the two to still be seen as two, rather than blended into a single intensity peak?

## Homework Equations

Ok so I have done question son diffraction gratings before, but all straightforward, and using the equation d*sin theta =m*lamda

## The Attempt at a Solution

i have worked out theta to be 32.6 degrees, and (not sure if this is right) but used the equation for double-slit diffraction: y=m*lamda*D/d and worked out the spacing between the maximum and the first minimum, y, to be 0.05389m. Is this at all on the right track or am I totally lost?

I fear the latter. Any help much appreciated!