The observation hole allows us to analyze the scene from a complementary point of view. If a source is not observable from the hole because an object is interposed between it and the observer, this implies that the hole is in the shadow of the object generated by the source and that the corresponding zone of the screen is not being illuminated by that source.
The shadows will appear closer if the sources are placed closer to each other. They can even overlap if an observer is located at the intersection of both shadows, from where none of the sources is visible. This zone is darker than individual shadows, from where one of the sources, but not the other, would be visible. This observation can be used as a first approximation to the concept of penumbra, since the two point sources can be interpreted as elements of an "extended source".
Click and drag the two spotlights.
Click on 'Rays' to draw some rays and click on 'Hole' to look at the scene from behind the screen.