This is a great article. Thanks for the effort. Just one thing; can you explain absorption, please? Is that just the fact that darker materials absorb more light?
As Alan says in the article, ‘“absorption” […] means that light is absorbed over distance as it travels through the object.’ So you can make an object darker by increasing the material's attenuation of light going through it, but it'll also become darker when you keep the material the same, but scale the object to a larger size.
This is introduced together with refraction because, in that context, it's important for rays that go through the object, as opposed to bouncing off the surface. When you look at diffuse reflections, darker materials also technically absorb more light penetrating through the surface than lighter ones. The difference is the scale at which the effect becomes significant enough that you want to spend computations on simulating it. A piece of rock doesn't change brightness when you scale it up — unless you're looking at grains of sand. With clear water it's the opposite: You don't need absorption to render a cup of water, but you do need it to render a lake.
This is introduced together with refraction because, in that context, it's important for rays that go through the object, as opposed to bouncing off the surface. When you look at diffuse reflections, darker materials also technically absorb more light penetrating through the surface than lighter ones. The difference is the scale at which the effect becomes significant enough that you want to spend computations on simulating it. A piece of rock doesn't change brightness when you scale it up — unless you're looking at grains of sand. With clear water it's the opposite: You don't need absorption to render a cup of water, but you do need it to render a lake.