Fresnel Effect

http://www.3drender.com/glossary/fresneleffect.htm

http://kylehalladay.com/all/graphics/2014/02/23/Fresnel-Shaders.html

http://en.wikibooks.org/wiki/GLSL_Programming/Unity/Specular_Highlights_at_Silhouettes

http://en.wikipedia.org/wiki/Schlick's_approximation