Final answer:
The mountain in the painting would be assumed to be further away than the tree due to the use of atmospheric perspective, a common technique in landscape paintings that depicts distant objects as more hazy and less detailed.
Step-by-step explanation:
When an artist paints a scene where a mountain lacks detail and is out of focus, and a tree is sharply defined and bright green, it suggests the use of atmospheric perspective. This technique helps to create the illusion of depth in a painting by manipulating color and clarity to imply distance. Since the mountain appears lighter and grayer compared to the bright and detailed tree, the viewer might assume that the mountain is further away than the tree.
This perception is due to the tendency of distant objects to appear more hazy and less detailed in comparison to closer objects in our field of view. These visual cues mimic the natural interaction of light with the Earth's atmosphere, thus giving a credible depiction of space and form within the artwork. This can be observed in many landscape paintings, where the elements in the background often appear less distinct than those in the foreground.