Light.
Incident light and reflected light. We Photographers see both. Want to see like a photographer? There are some more things about light than qualities of hardness, softness, color and direction. How does it work? It falls and it reflects. And being shy light tends to fall off quickly into the shadows with alacrity. One would never really think this thin slice of photons between 450nm to 750nm would have such personality. But before pursue light being chased by its own shadow it is helpful to step back and explore light that is illuminating and light that is reflecting.
Incident and Reflected. One could argue that ALL light is all reflected because that is what we see; and that is right. Nevertheless it is most useful to know the incidence of light, its source and its path. And how this light informs our subjects and work as photographers. You can probably have a career in photography without going into this theoretical objectification. However it would be a loss to miss this sensual observation of light’s power, nuance, and beauty. We are born out of darkness in the womb. It is said when we pass away we move into the light. Why not not make the most of observing light as we travel day to day, day to night and dark to light? Now, google this: To meter or not to meter? ~TEU