English: Prototype metallic lens antenna for 6 GHz microwaves, developed at Bell Labs in 1946 by Winston E. Kock, shown standing next to it. It consists of a 10 ft × 10 ft vertical lattice of parallel metal strips in the form of a Fresnel lens. Microwaves from a feed horn behind the antenna pass through the lattice, which focuses them into a parallel beam. The spaces between the strips act as waveguides. It functions similarly to a convex optical lens, slowing the velocity of the waves passing through the center, while increasing the velocity of the waves through the periphery. However in a converging optical lens the glass slows the speed of the waves, so the lens is made thicker in the center than the edges. In the microwave lens the waveguides actually increase the speed (phase velocity) of the microwaves, and thus have an index of refraction less than one, so to make a converging lens it must have a concave shape, thicker in the peripheral regions and thinner in the center. This antenna was used in the first microwave relay stations built by AT&T's Long Lines division in the 1950s. Lens antennas were also used in military radar.