Nd
Simply put, Nd should be regarded as the IOR (As mentioned in the Maxwell Materials page, the Nd is the index of refraction ) of the material. The name Nd is used because it is the common way to denote an index of refraction that has been measured at the wavelength of 583 nm for a certain material. You can find a list of common IOR values here. The Nd is important not only for transparent materials but for all materials as it defines both surface reflections and the refraction of light inside the object.
The Fresnel Effect
Before explaining how Nd influences the look of a material, it is important to understand the “Fresnel effect”. This effect states that the strength of reflections on a surface is dependent on the viewing angle. For example, if you look straight on at your monitor screen, you will see very weak reflections, but if you look at the screen at an angle, the reflections will appear stronger. It is named after the french physicist who described this effect.
The same plastic material with Nd=1.6 (left) and ND=30
...
From the images above you can see that raising the Nd increases the effect of the Reflectance 90° color, while the Reflectance 0° color (in this case set to nearly black) has less and less effect when increasing the Nd. Of course the value of the Reflectance 90º color will also affect the maximum strength of reflectance. For example, you can set a high Nd but set the Reflectance 90° to a dark color. The result will be a surface which is reflecting equally, no matter the viewing angle, but the reflections will appear weaker.
For opaque shiny materials (low roughness), an Nd of at least 3 is recommended if you want them to be reflective. Metals have an To mimic a metallic material the Nd needs to be even higher Nd. The Nd setting can go as high as 1000. For a mirror-like surface it is usually enough to set the Nd to 100.
Nd and Roughness
The Nd has no influence on the look of the material when working with Lambertian materials. Lambertian is a special mode which makes a surface reflect all light in a diffuse way . A Lambertian material has a roughness 100. (its roughness is 100). It is the highest roughness a surface can have. It is also important to remember that the Nd has less influence on the reflectance of an object the higher you set the Roughness (See the the Surface Properties chapter page for more details on the Roughness parameter). The smoother a surface is (low roughness), the bigger the effect of Nd on the surface will be. You will see the biggest changes to the reflectiveness of the object as you change the Nd, when the surface roughness is low. With transparent materials the Nd also controls the amount of refraction. Seawater for example has an Nd of about 1.333. You can find common Nd values for transparent materials on the internet.
...
You should turn on Force Fresnel when you are working with untextured refl. 0° and refl.90° channels to create more realistic metals or other shiny materials. Force Fresnel will then ensure a correct reflectance across the entire surface, even if you set a very dark refl. 0° color.
K parameter
From a physical point of view, the index of refraction is not just a plain number. In fact it is a number derived from a complex calculation to define the refractive index at one particular wavelength. This is the calculation:
...
The Nd represents the refractive index, which is the well-know concept of Index of Refraction we often use.
The K is the extinction coefficient: the amount of absorption loss when an electromagnetic wave propagates through a material. This is usually confused with the Abbe, but it is not related to this. K is related to the extinction of the wave.
The use of the K value is optional. In most situations it is enough to just use the Nd value. Only in specific situations where the extinction effect is important, is it necessary to use the K parameter to get a more precise result.
The values of the extinction coefficients are obtained from measurements in laboratory, and are also included in the IOR files.