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In 1670 Isaac Newton demonstrated the composite
nature of sunlight when he sent a carefully collimated sunbeam
through a prism, which spread out the light into a rainbow of
colors; by sending a beam of single color through a second prism
(with no further spreading) Newton showed that the color was not
being imposed by the prism but was intrinsic to the light itself.
Now physicists using the Advanced Photon Source at Argonne National
Lab, in Illinois, have spread out a beam of X-rays (which are, after all, just a
more energetic version of visible light) into a rainbow of colors.
Trying to reflect X-rays from a surface is difficult because X-ray
wavelengths are some 10,000 times shorter than those for visible
light. Glancing reflection of only a few tenths of a degree is
normally possible, and even then the beam of X-rays will suffer very
little wavelength-dependent spreading. However, another phenomenon,
Bragg diffraction, allows for scattering of X-rays from a crystal
through large angles; in this case the incoming X-rays scatter not
merely from a top layer of atoms in the crystal but from numerous
atomic planes. Furthermore, if the atomic planes are not parallel
to the crystal surface the diffracted X-ray beam will be spread out
prismatically into a range of component wavelengths (or colors).
 (a) White light at visible wavelengths gets spread out into its constituent colors by a prism. (b) An X-ray beam is spread out into its constituent "X-ray colors" by Bragg diffraction from a crystal whose atomic planes are oriented at an angle to the surface of the crystal. Image: AIP.
In
the Argonne experiment an incoming beam of 9-kiloelectronvolt X-ray photons with
angular spread of only 1 micro-radian (two-tenths of an arcsecond)
was backwards scattered and spread out into an X-ray rainbow with an
angular dispersion of 230 micro-radians.
Argonne physicist Yuri
Shvyd'ko says that his rainbow
is not just a novelty but will have many practical applications in
X-ray optics. These include compression of X-ray pulses in time and
the development of X-ray monochomators (which fashion X-ray beams of
pure wavelength, or color) and much higher-resolution X-ray
spectrometers.
Source: AIP
Related Links:
Shvyd'ko et al., Physical Review Letters, 8 December
2006
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