>>1014831
>How do I know if I learned it?

The degree to which you actually understand this is tied to how well you could hold a unprompted talk on the subject accounting for the way nature operates and why we are seeing what we are seeing upon looking at a scene before us.

This splits into three major parts

1) how well you can explain the core phenomena in the electromagnetic environment you're in, which is a fancy sounding way of saying what
is emitting photons/light? Where is the light visible to us from any point in the scene really coming from? How does it end up where it is in the scene?
Is it harsh direct from a directional lightsource or softened/diffused thru a filter, is it ambient/bounced off other things in the environment?

2) What actually happens with a photon when it interacts with a spot on a object in the scene? This comes down to how an advanced 'BRDF' you can account for.
The 'Bi-Directional Distribution Function' is a term you'll hear a lot. What does it mean? It means that the outcome of what happens at a spot on the object a photon touches
depends on the angle between you the observer and the direction of the surface and the direction towards the lightsource that's the 'Bi-directional' bit.
Objects look different depending on where you place your lightsource relative to your vantage point or how you orient the surface in your view.

The 'Distribution Function' then is how much physical phenomena in the real world do you know/understand the behavior of once a photon interacts with it?

If you understand the Blinn/Lambert which is the most legacy basic BRDF we have you have an entry point to start answering the questions of Op's image authoritatively.
If you understand up to the current era PBR model you could comprehensively account for phenomena and gave about as good a understanding as anyone outside active research currently has.

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Light, Coreshadows, Midtones, Reflected Light