A sliver of silver, which is totally opaque, when sandwiched between two transparent layers of another material becomes transparent, according to US researchers. Thin silver films are normally completely opaque but can be made to transmit light with 35% efficiency through an effect analogous to quantum tunneling. If a light ray passing from glass to air strikes the interface at a sufficiently shallow angle, it is reflected internally. However, some of the electromagnetic field strays across the boundary between the two materials as a so-called evanescent wave. If this wave encounters another block of glass a short distance away, a true light wave with reduced intensity appears in the second block. Normally, complete transmission does not occur, but Ian Hooper and colleagues at the University of Exeter, UK, have devised a system in which two glass prisms coated with a thin layer of zinc sulfide sandwich a 40 nanometer layer of silver between them. The effect demonstrates the quantum nature of light but could also have applications in opto-electronics.