What is behind the curtains?

Object A:  Initialize      Object B:  Initialize    

Object C:  Initialize     Object D:  Initialize    

Four regions are hidden by a curtain.  The light sources are drag-able.  The arrow represents an object, the fainter arrow represents its image.  

a)  What is behind each curtain?

b) Rank the objects in terms of their focal lengths from smallest to greatest.


Physlet Problem by Melissa Dancy and Wolfgang Christian