As I slowly begin returning to painting landscapes the question of light arises. For most of my artistic career I have not been that interested in light in paintings, though I enjoy it in the works of other artists. However in landscape painting light is very important, so lately I've been examining the use of light by various landscapes painters, all of whom would be called "realists." What I found in closely studying their paintings is that they are not strict and mathematical, if you will, about where the light hits. In fact in many realist paintings – those of Andrew Wyeth come to mind – the light is very natural looking at a glance, but then upon closer examination is highly invented. I find nothing wrong with artists taking this liberty.
The key to light in landscape painting seems to be to make an effort to simply include it in the work, and not obsess over where everything lines up and how it strikes each object in a uniform way... almost like a Bob Ross painting. Each object typically has a light area, a mid value, and a dark area. Get that right and the painting will take care of itself. The key is not worry so much about literally reproducing the exact way light strikes each object in succession. If one can do the and do it well, fine. But it is not necessary. The mathematics of how the light strikes is secondary to the presence of light itself.