  Microfabrication of graphene devices used in many experimental studies
currently relies on the fact that graphene crystallites can be visualized using
optical microscopy if prepared on top of silicon wafers with a certain
thickness of silicon dioxide. We study graphene's visibility and show that it
depends strongly on both thickness of silicon dioxide and light wavelength. We
have found that by using monochromatic illumination, graphene can be isolated
for any silicon dioxide thickness, albeit 300 nm (the current standard) and,
especially, approx. 100 nm are most suitable for its visual detection. By using
a Fresnel-law-based model, we quantitatively describe the experimental data
without any fitting parameters.
