Thin lens positive if the surface is concave, and negative if it is convex. This is an arbitrary sign convention; some authors choose different signs for the radii, which changes the equation for the focal length. Certain rays follow simple rules when passing through a thin lens, in the paraxial ray approximation: By tracing these rays, the relationship between the object distance "s" and the image distance "s′" can be shown to be which is known as the thin lens equation. In scalar wave optics a lens is a part which shifts the phase of the wave-front. Mathematically this can be understood
Compound refractive lens Compound refractive lens A Compound refractive lens (CRL) is a series of individual lenses arranged in a linear array in order to achieve focusing of X-rays in the energy range of 5-40 keV. They are an alternative to the KB mirror. For all materials the refractive index for X-rays is close to 1, hence a single conventional lens for X-rays has an extremely long focal length (for practical lens sizes). In addition, X-rays attenuate as they pass through a material so that conventional lenses for X-rays have long been considered impractical. The CRL gets its reasonably short focal length, on