Collimating light from diffraction grating

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Homework Statement

Consider the following, A collimated light source impinges on a plane diffraction grating and diffracts via the well known diffraction equation. The diffracted light is then collimated by a 1 metre focal length lens (good quality one). It is experimentally verified that the line focus(fourier plane) is at 0.7m if the distance between the grating and lens is 0.3m.

Now is the time for a silly question. I've tried to use thin lens equation 1/image + 1/object = 1/focal length but this clearly doesn't solve the problem above. Any ideas which equation I should be using? I assumed diffraction grating as a point source of light with some divergence. I know I am making the mistake somewhere but not sure.

I am using a plano convex lens. Does this matter?

Edit/ I've added the diagram now to make it clear. what is the relationship between x an y in this configuration.

Homework Equations

I think thin lens equation and grating equation are well known already.

The Attempt at a Solution

As mentioned already , the thins equation doesn't correctly predict the position of the line focus(fourier plane).

 

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Any ideas chaps?