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A little more on Wavelength and Monochromatic Radiation

X-rays belong to the short wavelength end of the electromagnetic spectrum. We can think of them as sinusoidal waves of electromagnetic vibration, travelling at the speed of light (see below).

wavelength illustration

The most important parameter is the wavelength, illustrated and denoted by λ, which is the distance between successive crests (or troughs) of the travelling wave. If a source of radiation produces waves, all with the same wavelength, it is said to be a monochromatic source; alternatively a source producing a wide range of wavelengths is said to be continuous, white, or Brehmsstrahlung.

By far the most common monochromatic radiation used to the present time has been the characteristic (so-called Kα) X-rays produced by a laboratory copper X-ray tube source (further details will be given later in the course). This produces X-rays with a wavelength around 1.54 × 10−10 m (metres), or 0.154 nm (nano-metres), or 1.54 Å (Ångström). The preferred distance unit of crystallographers is the Ångström (1 Å = 10 −10 m) and we will usually use this.

When such rays travel through matter the vibrating electric/magnetic fields interact with atoms, and are scattered by the constituent atoms. A measure of the energy, E (and therefore also the penetrating power), of X-rays is provided by the reciprocal of the wavelength λ:

E  (keV)  ≈  12.4
λ  (Å)

Thus the shorter the wavelength, the more energetic the X-ray and the greater its penetrating power.

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© Copyright 1997-2006.  Birkbeck College, University of London.
Author(s): Paul Barnes
Tony Csoka
Simon Jacques