Focal length of diffraction grating for recip. dispersion

[magnesium12]

Homework Statement

What would the focal length be of a diffraction grating if the set up is a laser placed 1 m from the 0 order diffraction?
Would it be the distance from the grating to the laser or the distance from the grating to the image?

Homework Equations

1/f = 1/o + 1/i
Rd = [Df]^-1
D=[sina + sinb]/λcosb
sina = 0

The Attempt at a Solution

 

[BvU]

There is no such things as a focal length for a diffraction grating. What makes you think there is ?

 

[magnesium12]

I assumed there was because I need to calculate the linear reciprocal dispersion of a first and second order diffraction and the formula is R_d = [Df]^-1 where f is focal length (in my notes).
Is it possible to find a linear reciprocal dispersion for a diffraction grating at all?

 

[BvU]

Perhaps you should start with a complete and concise problem statement with all the variables clearly described. And then the relevant equations should establish relationships between those variables. By the time that's sorted out the problem is a mathematical problem. In the solution attempt you try to manipulate the relationships until the quantity that is asked for is expressed uniquely in terms of the given data.

There is such a thing as dispersion for a grating, so don't despair