\section{Refractive lenses}
\label{s:lenses}
\index{Optics!Lens\_parab}

An X-ray refractive lens, often referred to as a Compund Refractive Lens (CRL), is a fairly new type of device, which has gained popularity in
the last few years. An early study showing the feasiblity of such devices may be found in~\cite{snigirev1996}. As the refractive index of X-rays
is $n\approx1$ a number of lenses, stacked together is usually necessary to bring the focal length to practical values. 
\MCX includes a few lens components, which all have slightly different charactertics.

