Post your articles online and promote your website for free! Boost your sites ranking by linking it from the ArticleZap database! Free Article directory and instant translation publishing!
Compton effect, increase in wavelength of X-rays and other strong electromagnetic radiations that have been scattered by electrons; it is a principal way in which this penetrating radiant energy is absorbed gradually in matter.
The Compton effect has proved to be one of the cornerstones of quantum mechanics, which accounts for both wave and particle properties of radiation as well as of matter.
After a thorough investigation of X-ray scattering, the U.S. physicist Arthur Holly Compton successfully explained (1922) this phenomenon of wavelength increase by considering X-rays as composed of discrete pulses, or quanta, of electromagnetic energy, to which he applied the name photons. Photons have energy and momentum just as material particles do; and they also have wave characteristics, such as wavelength and frequency. The energy of photons is directly proportional to their frequency and inversely proportional to their wavelength, so that lower energy photons have lower frequencies and longer wavelengths.
In the Compton effect, individual photons collide with single electrons that are free or quite loosely bound in the atoms of matter. The colliding photons transfer some of their energy and momentum to the electrons, which in turn recoil. In the instant of the collision, new photons of less energy and momentumare produced that scatter at angles the size of which depends on the amount of energy lost to the recoiling electrons.
Because of the relation between energy and wavelength, the scattered photons have a longer wavelength that also depends on the size of the angle through which the X-rays were diverted.
If the wavelength of the incident photons is expressed as A (lambda), that of the scattered photons as A’ (lambda “prime”), the mass of the electrons as m, the Compton effect or shift in wavelength may be mathematically stated as a trigonometric function (cosine) of the scattering angle 4) (phi).
Two universal constants also enter into the mathematical statement: c, the velocity of light, and h, called Planck’s constant, which characterizes all physical phenomena involving small particles such as atoms and photons. The increase in wavelength of the scattered photon over that of the incident photon, accordingly, may be expressed as the difference: A’ — A = (h/me) (1 — cos 4)).
In the above equation the factor h/me, having the dimension of length, is known as the Compton wavelength of the electron, which equals 0.024 angstrom. The Compton wavelength of any particle may be computed by substituting for m the value of its mass while at rest.
The factor 1 — cos 4) has a value of zero for 4) = 00, one for 4) = 90°, and two for 4) =
180°. Thus, when photons are scattered from electrons by the Compton effect, there is no shift in the wavelength of photons that are undeflected, an increase in wavelength equal to one electron Compton wavelength for photons scattered 90° or at right angles, and an increase in wavelength of two electron Compton wavelengths for photons deflected back in the direction from which they came.
The increase in wavelength or Compton shift, therefore, ranges from zero to two Compton wavelengths of the particle with which the photon collides and does not depend on the actual value of the photon wavelength.
Because X-ray photons have relatively short wavelengths (typically one angstrom) and “radio-wave photons” have extremely long wavelengths (centimetres to kilometres), the Compton shift is a detectable fraction of an X-ray wavelength but completely undetectable for radio waves.
