Why Does Moving the Microscope Objective Create a Second Image?

[Numen]

Homework Statement

I have a transparent plate (call the thickness t and refractive index n), it has a scratch at a point A. A microscope objective (M, positive) images the point A in A' at a distance D from the objective. If we move M a certain distance d down (without changing focus, meaning D is unchanged), there will be another sharp image of the scratch.

How come?

Homework Equations

Not needed?

The Attempt at a Solution

I was thinking that it might be some reflection on the objective that reflects the light back to the plate and back to the objective at the same point as before it was moved, and then refracted towards A'. I'm not sure if I understand the problem correctly... I think it is suggesting a second image somewhere else?

 

[Numen]

Ok, I tried illustrating the problem. Please check the attachment (hope it helps).