The intensity of the image for the object is reduced is some light path
were blocked (the whole image is still there). Select from drop-down
menu or drag to "block part of light path" button or drag lower end of the
black line to lower position to block part of light paths. The
intensity is not the same if part of the light paths were blocked. You
can change the slit position and its width with mouse drag.
Sample Learning Goals
There is a common misconception among students: Blocking part of
the lens surface would block the corresponding part of the image.