Answer:
It gets darker after etching when observed through optical microscope.
Step-by-step explanation:
This question deals with the study of the structures of different kinds of metallic alloys in a micro way that is to say METALLOGRAPHY.
So, ETCHING in METALLOGRAPHY simply means the application of chemical substance to show the microstructure of a metallic alloy.
What one will observe through optical microscope when grain boundaries are etched is that there will be darker appearance because a lot of light is scattered which in turn will give a darker appearance of the grain boundaries.