X-rays

X-rays have much shorter wavelength, or higher frequency, than even ultraviolet rays. While the intensity of a wave is proportional to the square of its amplitude (the E field amplitude for EM waves), if one considers the waves instead from their dual particle perpective, then the energy of each discrete photon (packet of energy) depends on its frequency through the relation

$$E= hf \\$$ (7.2)

where $f$ is the frequency of vibration, and the constant $h$ is an empirical constant of nature, required by dimensional analysis, and is called Planck's constant. Its numerical value is given by

$$h=6.62618 \times 10^{-34}Js \\$$ (7.3)

Thus an X-ray photon carries much more energy than that of visible light or ultraviolet radition, and while the latter might be able to disrupt a single chemical bond, X-ray photon can destroy many bonds and cause permanent damage to molecules. Thus even low intensity (i.e small number of photons) X-rays are dangerous because they can cause significant permanent damage which becomes apparent only some time later. The X-rays used for medical purposes are created by colliding fast-moving electrons with a heavy target. The rapid deceleration of the electrons produces high-energy electromagnetic rays in the X-ray range. X-ray imaging makes use of the photoelectric effect: When an X-ray photon strikes an atom, it ejects an electron with a kinetic energy equal to the difference between the X-ray photon energy and the binding energy of the electron. Now it turns out that small atoms which have mostly weakly bound electrons, are quite unaffected by high energy X-rays while the tightly bound electrons in large atoms have a much greater probability of absorbibg the X-rays. Thus tissue which contains mostly small atoms like C,H,O,N, is relatively transparent to X-rays while bone with its larger Ca and P atoms casts shadows in X-ray imaging.