Biomedical Engineering Reference
In-Depth Information
“Hence the difference between similar and equal things, which are
yet not congruent (for instance, two symmetric helices), cannot be
madeintelligiblebyanyconcept,butonlybytherelationtotheright
and the left hands which immediately refers to intuition”
[6].
W.Thomson(LordKelvin),whowasoneofmostimportantfigure
in physics at the end of the XIX
th
century, defined chirality more
precisely in the followingway:
“I call any geometrical figure or group of points, chiral and say it
has chirality, if its image in a plane mirror, ideally realized cannot
be brought to coincide with itself”
[7].
The interest of Kelvin for chirality is not surprising. He played
himself a critical role in the foundation of electromagnetism and
thermodynamics, and he was well aware of the work presented
in 1896 by M. Faraday on what is now known as the Faraday
effect, which is intimately related to optical activity and chirality
[8]. In this context, it is interesting to observe that like Pasteur
after him, Faraday also had fruitless attempts to establish some
relations between electricity, chirality, and light; it was a letter from
Thomsonin1845thatactuallyledFaradaytorepeathisexperiments
with a magnetic field and to discover nonreciprocal gyrotropy (i.e.,
magnetic optical rotation).
Remarkably, this description of Kelvin provides an operational
definition of chirality particularly suited to optics, as we illustrate
below. In optics indeed, since the pioneer work of Arago [9] in 1811
and Biot in 1812 [10], chirality is associated with optical activity
(naturalgyrotropy),whichistherotationoftheplaneofpolarization
of light upon going through a 3D chiral medium such as a quartz
crystal or an aqueous solution of sugar. The first mathematical
description of optical activity has arisen from the work of Fresnel in
1825 [11] who interpreted phenomenologically the effect in terms
of circular birefringence, that is, as a difference in optical index for
leftandright-handedcircularlypolarizedlight(writtenLCPandRCP,
respectively) passing through the medium. However, the intimate
relationship between optical activity and chirality became more
