  Reflection of light from surfaces is a very common, but complex phenomenon
not only in science and technology, but in every day life. The underlying basic
optical principles have been developed within the last five centuries using
visible light available from the sun or other laboratory light sources. X-rays
were detected in 1895, and the full potential of soft- and hard-x ray radiation
as a probe for the electronic and geometric properties of matter, for material
analysis and its characterisation is available only since the advent of
synchrotron radiation sources some 50 years ago. On the other hand
high-brilliance and high power synchrotron radiation of present-days 3rd and
4th generation light sources is not always beneficial. Highenergy machines and
accelerator-based light sources can suffer from a serious performance drop or
limitations due to interaction of the synchrotron radiation with the
accelerator walls, thus producing clouds of photoelectrons (e-cloud) which in
turn interact with the accelerated beam. Thus the suitable choice of
accelerator materials and their surface coating, which determines the x-ray
optical behaviour is of utmost importance to achieve ultimate emittance. Basic
optical principles and examples on reflectivity for selected materials are
given here.
