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On the window sill
Your answer is correct.
The rays of sunlight are refracted while passing through the window pane. This is the reason for the fact that they come together behind the window pane. If light hits the boundary of a medium at an angle, it will be deflected. Because of the outwardly rounded curvature it is refracted differently at every point. This results in the collecting of the light behind the pane. In the toolbox you can read about further specifications.
The leaves of the plants on the window sill were a few centimetres away from the pane. In case of water waves it is also necessary to have a minimum distance to see the light phenomena. To solve the brain-teaser, think about the way it is in case of water drops on the leaves and what kind of effects there are on the focussing of light.
