To anti-reflect a specific wavelength with one layer of
coating, ideally a compound would be used that has an index of
refraction that is midway between the indices for air and the
optical substrate. Additionally, the optical thickness of the antireflective
layer is usually chosen to be one-quarter wave. If
both of these criteria can be met, the theoretical reflection at
that specific wavelength is zero. There are practical
considerations that prohibit this in the visible wavelengths.