Figure 1
Alan Stonebraker, adapted from [1]

Figure 1: (a) The slab of empty space 0<x<L is optically cancelled by a slab of negative-index material in -L<x<0. (b) The same cancellation effect works with an object in 0<x<L, if the negative-index slab contains an anti-object. (c) A spherical shell b<r<c is optically cancelled by a negative-index shell a<r<b. If the core r<a is optically equivalent to a sphere of radius c, then this device is invisible. (d) The same as (c) but with an object in the cancelled shell b<r<c. The object is cloaked: both it and the cloaking sphere are invisible.