# Homework Help: Focal length of diffraction grating for recip. dispersion

1. Feb 25, 2016

### magnesium12

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
1/f = 1/o + 1/i
Rd = [Df]-1
D=[sina + sinb]/λcosb
sina = 0

3. The attempt at a solution

2. Feb 25, 2016

### BvU

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

3. Feb 25, 2016

### 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 Rd = [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?

4. Feb 25, 2016

### 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