Light runs backwards in time

The concept of a refractive index is familiar to every physicist: wine glasses sparkle, deep pools appear shallow and camera lenses focus sharp images. As every physics student knows, Snell's law relates the angles of incidence and refraction in materials with different refractive indices. However, my complacency was recently given a jolt by Sheldon Shultz, David Smith and co-workers at the University of California at San Diego who have made a material with a negative refractive index (D Smith et at. 2000 Phys. Rev. Lett. 84 4184).