The look of a material depends on what happens to light hitting that material. How much is reflected to our eyes and in what fashion. How this differs over wavelengths of color and over the angle at which we see the surface reflecting light.
All materials can be thought of as somewhat transparent, with a density, surface structure and sub-surface structure. This together with the thickness/depth of the material will determine how far light can get inside and how much of that light will find its way out again via sub-surface scattering or pass through. Transmissive materials, that let light through, always refract light (the light passing through is bent because of wavelength compression or decompression when going from one media to another) and are sometimes called translucent (when the light passing through them is also scattered).
At the surface level we get some absorbtion and reflections that are specular or diffused by surface scattering. On the sub-surface level we get internal reflections, more absorbtion and sub-surface scattering, often contributing to the overall visible diffused light.
As you probably know, white light contains light of all visible wavelengths, the entire spectrum of the rainbow. Light can be absorbed by a material depending on the makeup of that material on a microscopic level.
Here almost all of the light hitting the ball is absorbed, leaving a dark grey / black surface. A good example of this is soot. And black holes…
If some wavelengths of light are absorbed, and others reflected, the material will look colored by the reflected wavelengths, since that is the only light left that can reach our eyes. This is what we call subtractive color.
Reflections of light are often labeled like this:
- Diffuse (Light bouncing off a rough or powdery surface or a sub-surface)
- Specular (Light bouncing off a glossy surface in a parallel manner)
- Caustic (Specular reflection, focused by the curvature of the reflecting surface in a pattern visible on other surfaces nearby)
- Anisotropic (Semi specular reflections that are stretched out like those on a brushed metal)
The surface of this ball is rough, dry and powdery, the light bounces off scattered by the uneven surface so we cannot see any sharp reflections, and we call this a diffuse reflection.
The surface is glossy and polished, the light bounces straight off without much diffusion.
Hitting this material, the light gets partly diffused so we cannot see any sharp reflections on surfaces facing the eye. In CG this is commonly controlled by a glossiness, eccentricity or cosine power value / texture.
Specular reflections are mostly colored only by the incoming light, thus textured in grayscale, but there are many exceptions to this, mainly among metals, some fabric and metallic paint. Satin, Gold, Copper etc.
This look with sharp but not very intense reflections indicates a material with a transparent glossy coating, esentially being two materials, or a material being slightly transmissive with its surface polished to a high gloss. Some of the incoming light is bounced off the surface directly and the rest get either absorbed if it is not red or diffused if it is red.
Anisotropic reflections appear when the surface is quite reflective and covered in lots of tiny scratches parallel to each other. The reflection is stretched out, not along with the scratches, but across them.
How diffused the reflected light will be is not constant for all viewing angles, the intensity and sharpness of reflections are higher at angles closer to 90 degrees away from the eye.
A red light shines on this stone floor and the reflected light is mainly diffused. The intensity of this light is notably lower as we progress closer and look down, ending over the brightest area, looking straight down at the floor.
In a similar fashion, this window show mirror like reflections at an angle. As we get closer to standing straight in front of the window, looking in, the reflection intensity fades and we can look inside. This is commonly called the Fresnel effect, pronounced in french, without the “s”. A Fresnel equation tells us how much of the light is refracted in through the glass and how much is bounced off the surface as a reflection, based on the viewing angle and the refractive index of the glass. Light from the interior also has to be able to refract out in the same manner for us to be able to see the room inside. In many renderers this behaviour can be automatically controlled by specifying the refractive index (around 1.5 for glass), or the reflection part can often be manually set in a BRDF control, specifying a value for reflection intensity at a 0 degree viewing angle and another value for 90 degrees, as well as an exponent for a curve, interpolating the values in between.
Transmitted light is always refracted, and sometimes scattered. The scattering can occur on a surface level or on a sub-surface level, just like for the reflected light. The difference is that transmitted light escapes the material on the opposite side of where the light source is.
Transmitted light is bent because of wavelength compression or decompression when going from one media to another. This behaviour is called refraction. The wavelength of light gets shorter when it enters a more dense material and longer when entering a less dense one. The waves are longest in vacuum, and just slightly shorter in air. Here is a nice visualization of how this happens.
The light is compressed while in the water and gets bent at the surface, so the pen looks bent.
The light transmitted through glass and water in this image is clearly focused, making up a caustic refraction pattern on the wood.
The index of refraction differs over wavelengths, resulting in different wavelengths of light bending to different angles. This is called dispersion, and the amount of dispersion varies between materials. You have probably seen this effect on crystals or dispersion from a lens stored in a photo, called chromatic aberration.
The transmitted light is not just refracted, but also scattered so we call the leaves translucent.
All you see is light!
I hope you found the info in this post useful. I am likely to go back at some point and revise or refine it. Let us have a look at one more picture to wrap this up:
This image shows many different interactions between light and materials. Look for instance at the specular reflection from this flowerpot shining on the wood and white paint. It is not evenly distributed but focused in a wavy pattern because of the surface of the pot being slightly wobbly. Much like when satellite waves are focused with a parabolic antenna. This is called a caustic reflection. The pot also reflects diffused pink light, and the leaves some green. You can also see some sub-surface scattered light exiting the leaves of the plant.