Light mastery in design is one of the most important things to learn when learning UI Design. It definitely helps when you think of your objects as a real 3D “stuff” that you can play and interact with, to see how they react to the properties of light.
Imagine if you could get a cursor or touch a button, what would be your object under a light source? How would it be under two light sources? What we see on a 2D screen must have the same properties under real conditions.
My advice to anyone starting to learn Design is to pay attention to the amounts of light as well as the amounts of shadows in their prototypes.