Figure 1.

(a) Generic mechanism for the emission of electromagnetic radiation, including X-rays: an electron has an acceleration a in the transverse direction y and emits waves in the longitudinal direction z. (b) To exploit relativity and produce short X-ray wavelengths, we add a longitudinal velocity v close to the speed of light. (c) A practical way to implement the mechanism (b), using a ‘bending magnet’ (Margaritondo, 1988 ▸) with a magnetic field of magnitude B _x in the transverse x-direction, which causes a Lorentz force along y.