The elements of light:
The center light, also know as direct light, is the plane (or planes) of the form facing directly towards the light source.
The highlight is actually a reflection of the light source. Generally, the brightest part of the form, the highlight will move to another location if the camera or viewer also moves. This is because of the law of reflection, which we will discuss at a later time. For now, it's enough to know that the angle at which the light hits the surface (the angle of incidence) is equal to the angle of reflected light (angle of reflection).
The area of the form that is exposed to the light is made up of the light midtones, a tonal gradient ranging from the center light to the core shadow. As a rule of thumb, the light midtones never get darker than the dark midtones on the form shadow.
The elements of shadow
The core shadow is usually the darkest part of the form, bordering the light side and the dark side. Only present if there is bounce light coming from another object or surface, the core shadow is the area least affected by any light source, including bounce light. Even if the core shadow can't be observed, most artists choose to create one anyway, as it implies the form very effectively.
Form shadow and dark midtones
The planes of the form facing away from the light source make up the form shadow. The form shadow is usually a gradient of dark midtones ranging from the core shadow to the edge of the from shadow. Though, if there is no bounce light affecting it, the form shadow might appear as a solid black mass.The form shadow may be affected by a few elements of reflected detail depending on how reflective the form material is and how bright and reflective nearby objects or surfaces are. Typically, we might see bounce light pushing the dark midtones up a little. We may also see a reflection of the cast shadow as well as reflections of other nearby objects.
It's important to know that most cast shadows are made up of three or four parts depending on the light source. If the light is a single point in space then we will only see an umbra (Latin for shadow), a simple cast shadow, solid looking with very little, if any tonal change. If the light source is non-point, meaning the light source is coming from an object with a certain surface area then we see some interesting variations in the cast shadow. As well as an umbra we also see a penumbra and less commonly, an antumbra. We'll need a visual diagram to talk more about umbra, penumbra and antumbra. We'll do this in a follow-up tutorial.
The occlusion shadow is the area that is least affected by light and therefore should be rendered the darkest, darker than the core shadow even.