, Bragg reflection provides us with of selecting photons of a particular wavelength from an incident beam containing a mixture of wavelengths. Since the equation is satisfied (for a particular angle of reflection) only for a single value of l This equation is known as Bragg's law, after the English physicist W.L. , where d is the interplanar spacing, so the condition for so-called Bragg reflection of the x-rays is: From geometry of the right-angled triangles ABD and BCD, AB = CD = d sin q Where n is any integer) the phase at points C and D on the wavefront of the reflected rays will also be the same and the two rays will combine constructively at the detector (e.g. If this extra distance is an integral number of wavelengths (AB + BC) further than the x-rays reflected from plane A. The x-rays reflected from plane B must travel an extra distance On a wavefront which is perpendicular to the incident direction. įor a parallel incident beam, the phase is the same at points (A and D in Fig. However this reflection takes place only for a particular angle of incidence, such that x-ray beams reflected from successive planes of atoms have their electric and magnetic fields in phase. In fact, the x-rays can be 'reflected' from the atomic planes, as if the latter behaved like a mirror

Photon energy or wavelength) by the nuclei of the atoms. The x-rays penetrate through many layers of atoms but at each layer there is a small probability of the x-rays being scattered elastically (with no change in Since atoms in a crystal are spaced regularly by a few hundreds of pm (some tenths of a nanometer, 1 nm = 10^-9 m) a crystal can be used as a kind of 3-dimensional diffraction grating to separate x-rays of different wavelengths.

Within a few years it became clear that x-rays are another form of electromagnetic radiation, with wavelengths typically some tens or hundreds of picometers (1 pm = 10^-12 m). Within months, the first medical radiographs were produced, showing bones (which absorb x-rays more strongly) as darker areas on the photographic emulsion. He accelerated electrons (produced in vacuum by thermionic emission from a heated filament) towards a positive metal target (the anode) by applying several thousands of volts, and found that a very penetrating form of radiation is produced at the anode X-rays were discovered in 1895 by Wilhelm Roentgen.

Figure references are to the second edition of Modern Physics by Serway, Moses and Moyer (Saunders, 1989).

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