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*Yoda*
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Before explaining the material parameters used in the Maxwell   material system, it is important to have a basic understanding of what   light is, how it interacts with materials and why a material looks   shiny, dull, transparent etc. Please review this information as it will   make the parameters in the Maxwell material editor much easier to   understand.
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h2. What is light?\\

Visible light is really a very  small portion in a range of  electromagnetic radiation. This radiation  travels in waves of different  wavelengths. The difference in wavelengths  (the "tops" of each wave)  is what makes the difference between blue,  red, gamma rays, x-rays,  radio waves etc.
"White light" is a  combination of all the colors in the visible light  spectrum. When we  perceive an object as red for example, what really  happens is that white  light falls on a red surface, and all the  wavelengths except those that  give red light are absorbed by the  material. Only the red portion of  the spectrum is reflected back.
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h2. Diffuse -- Specular & Reflected light\\

We  see an object because light is reflected from its surface into  our  eyes. That is, ALL light is reflected light. This may sound  confusing at  first because it has become common in renderers to refer  to reflected  light as specular (or sharply) reflected light.
In the real world  light does not have a separate "diffuse" or  "specular" part. So what  makes a surface look dull, or mirror like? It  is the smoothness of that  surface.
A surface which is not very smooth has tiny imperfections  which scatter  the light in all directions, thus creating a very  "diffuse" reflection  of its environment. So it is the reflection of  light from an uneven or  granular surface, resulting in an incoming light  wave being reflected  at a number of angles.
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A surface which scatters almost all light in a chaotic, diffuse way   is called a "lambertian" surface, such as the red ball on the left.   Lambertian reflectance means that light falling on a surface is   scattered in such a way that the apparent brightness of the surface is   the same, regardless of the observer's angle of view. Or else: the   surface's luminance is the same regardless of angle of view. Many rough   surfaces, such as unfinished wood, exhibit lambertian reflectance.
A  surface which scatters almost all light in a chaotic, diffuse way is   called a "lambertian" surface, such as the red ball below (F. 01).   Lambertian reflectance means that light falling on a surface is   scattered in such a way that the apparent brightness of the surface is   the same, regardless of the observer's angle of view. Or else: the   surface's luminance is the same regardless of angle of view. Many rough   surfaces, such as unfinished wood, exhibit lambertian reflectance
It  is important to note that because very smooth surfaces reflect light   perfectly, they reflect much less of their own color. This is visible  in  the render (See above F.02), where the ball is still tinted red, but  as  it is a tinted mirror, its own color shows much less. There are a  few  exceptions however and one of them is metals. Even if a metal is  very  smooth, it still reflects its own color much more.
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h2. Transparency\\

An  object becomes transparent when light does not stop at the  surface, but  goes through the object and out the other side. When light  passes  through a material, it slows down because the material is  denser than  vacuum. Because of this change in speed, the light is bent,  or  refracted, when going from vacuum (or air) into the material. This   refraction occurs whenever light changes speed, so it happens when it   moves from one material to another with different densities. The   refraction is also what causes "caustics": concentrated pools of light.
In  the render below (F.03), it is refraction that makes the magnifying   glass distortion, and creates the caustics pattern. Different materials   make light slow down more or less, relative to the speed of light in  air  or vacuum. This difference between lightspeed in vacuum <-->   lightspeed in medium, is specified as the index of refraction (IOR,  also  called Nd) for that particular medium.
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F.01                                                                                        F.02
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A  smooth surface however reflects light uniformly and creates a very   sharp or specular reflection of its environment. Specular reflection  is  the perfect, mirror-like reflection of light from a surface, in  which  light from a single incoming direction is reflected into a single   outgoing reflection, as for example with a mirror.
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F.03
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h2. The Fresnel Effect\\

The Fresnel effect is the apparent  increase/ decrease of a surfaces  reflectance based on viewing angle. For  example, if you look at your  monitor screen straight on, the monitor  glass shows almost no  reflection, but if you look at it from an angle  almost parallel to the  glass, it has become very reflective. The Fresnel  effect is dependent  on the Nd of the material. The higher the Nd, the  more reflective the  material becomes at ALL angles, so the Fresnel  effect diminishes -- the  material becomes equally reflective at all  angles. It is important to  understand this effect in order to create  realistic materials. A  plastic material for example should not have the  Nd (Fresnel) set too  high or it would become unrealistically reflective,  even when viewed  straight on. It would behave more like a metal than a  plastic.

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