Microstructure of Annealed Brass: Contrast Explained

[temaire]

Here is an optical microstructure of annealed brass:

[PLAIN]http://img576.imageshack.us/img576/7955/annealed.png

Where does the contrast between grains come from?

 

[Mech_Engineer]

The molecular structure is oriented at different angles in each grain, and therefore reflects light differently.

 

[RacinReaver]

It comes from your etchant attacking different parts of the surface of your material at different rates.