The Fresnel node combines Reflect and Refract with the ratio automatically calculated based on index of refraction and angle to viewer. It only performs the correct calculation for a material that is 100% clear at 0 degrees and 100% reflective at 90 degrees - i.e. a perfect material. In real life things like clear glass, clear undisturbed water, and diamond are examples of things that the Fresnel node correctly models. Things it does not model correctly are colored glass, water with particles in it, or colored gems such as ruby or emerald.

The Fresnel effect still comes into play with all materials, but if there is any diffuse reflection, or if the refracted light is colored in any way, then we must build a different calculation using the nodes for Reflect, Refract, Diffuse, and a lot of math nodes. That is why I say you can use the Fresnel node for certain things, because it is correct and convenient. In other cases, when it is incorrect, you must give up the convenience and build a different (but similar) set of rules using more nodes.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)