In the March of 2000, scientists at the University of California at San Diego announced their production of the first “Left-Handed Material.”¹  These materials are exciting because of their unusual optical properties; in a left-handed material, light seems to propagate opposite the direction of energy flow.  This leads to a negative index of refraction and reversed Doppler shift for radiation!  The materials were called "left-handed", because the electric and magnetic fields propagate in a direction given by a left-hand rule.  (Conventional materials follow a right-hand rule, as described in the Introduction to Waves and Electromagnetic Radiation module.)  But "left-handed" is used to describe a different set of properties when applied to particles (see this page of the Particle Adventure), so some² prefer to identify these materials as "negative index" materials.

Snell’s law has long been used to calculate angles and refractive indices for refractive materials.  What we didn’t know until recently, however, is that these indices of refraction can be negative!  As light enters a left-handed material from a right-handed material (a material we are already familiar with like glass or air) the light will refract, but the refracted ray in a left-handed material will be a mirror image of the refracted ray we typically see in right-handed materials.  (See picture.)

Refraction is not the only effect where left-hand materials give an unexpected result.  Light traveling toward a detector through normal (right-handed) materials appears “blue-shifted” because of the Doppler effect.  As the waves approach an object, they bunch up, decreasing the wavelength as measured by the object.  But in a left-handed material, light traveling toward an object is red-shifted, appearing to have a longer wavelength!  For another explanation or just looking for more information about the Doppler effect, refer to this site (from the Glenbrook High Physics Classroom) for a detailed description of the Doppler effect in sound, or to this site (from the notes for Michael Bolte's course on Stars ) for a summary of the Doppler effect for light.

 To this date, no one actually knows why the materials act the way they do.  The only form of explanation they have is that the material has a negative electric permittivity and negative magnetic permeability, which could explain why the Doppler shift and Snell effect are inversed.  Unfortunately, they have yet to explain why the material’s permeability and permittivity would be negative.  The links below provide more information about this exciting new class of materials.  You might want to bookmark footnote ³; that page will be updated as more developments arise.

References:

1	http://ucsdnews.ucsd.edu/newsrel/science/mccomposite.htm
2	http://www.physicstoday.org/vol-57/iss-6/p37.html
3	http://physics.ucsd.edu/~drs/left_home.htm