The material structure explained here is almost perfect, except for the cases when the medium scatters rays. That’s what usually happens with diffuse surface: the light goes inside the object to a very small depth, then randomly reflects and goes out to a random direction. That’s how we get diffuse surface in real life. Some materials do not reflect rays inside themselves, like glasses or clean liquids, while others reflect rays immediately like metals or plastics. However, there are materials that let some rays go inside to some amount and then reflect them back. These are: wax, milk, marble and much more. The effect itself is called subsurface scattering and sometimes helps to significantly improve the visual appearance of the scene.
Let’s start with an empty scene and add a prism there:
The key requirements of subsurface scattering to work is that the object must have translucent medium (as otherwise there is no way for rays to scatter inside the medium). So let’s turn the “diffuse” block off and enable the specular one instead, making sure its IOR N parameter is set to 1.5. Then switch the material medium type to manual with the same IOR:
At the moment we have a simple glass prism. Now it is time to tint the medium. Change the Absorption parameter to light beige color (RGB 254, 252, 233) and set Attenuation to “2”:
Now we have a prism of beige glass and it is time to enable the subsurface scattering effect, just check the box at the bottom of Main parameters group in materials editor panel:
See the difference? When we enable SSS (short for subsurface scattering) rays can’t go directly through the medium anymore. They scatter inside and run out in various directions, so we see that noisy prism that is now slightly similar to wax. Let’s have a look at the parameters:
So once again, the key options of SSS are Absorption and Attenuation that are defined as medium parameters. Then you enable SSS and adjust Scale and Color parameters to get the effect you need. Subsurface scattering is a very complex effect and it may take time to master it.
Also note that SSS effect is quite noisy and using it will significantly increase the rendering time, so use it when you are sure you need it and ready to wait longer.