Electrical permittivity and magnetic permeability are concepts that are deeply embedded in electromagnetism. The electrical permittivity of a material determines its response to an applied electric field, while the permeability summarizes how it reacts to an applied magnetic field. Together the permittivity and permeability determine how the material responds to electromagnetic radiation of all wavelengths. Indeed, the speed of light in vacuum can be defined as c = 1/(E0µ0)1/2, where E0 is the permittivity of free space and µ0 is the permeability.
When a beam of light enters a material it usually slows down and changes direction. The degree to which this happens is determined by the refractive index of the material. The refractive index, n, is defined by the simple equation n = (Eµ)1/2, where E is the permittivity of the material (relative to the vacuum) and µ is the relative permeability.
In the September issue of Physics World, John Pendry of Imperial College, UK, reveals how negative E and µ lead to a variety of surprising new physics and the possibility of applications in communications, electronics, optics and medicine.