Comparison of Gouraud and Phong Shading:

Phong shading needs more calculations, but produces better results for specular reflection than Gouraud shading in the form of more realistic highlights.

Let the specular reflection term in Eq.:

                                                       cosⁿ (s)

If n is large (the surface is a good smooth reflector) and one vertex contain a very small value of s (this is reflecting the light ray in the direction of the observer) whilst the rest of the vertices have large values of s - a highlight takes place somewhere on our polygon.

With Gouraud shading, nowhere on the polygon may have a brighter colour (that means higher value) than a vertex so unless the highlight take place on or near a vertex, it shall be missed out altogether. While it is near a vertex, its effect is spread over the whole polygon. With Phong shading though, an internal point may indeed have a higher value than a vertex and the highlight shall occur tightly focused in the (approximately) correct position.