Last week's episode of Futurama showed Fry becoming a cop and dispensing justice Tron-style. During a high-speed motorcycle pursuit, he and his partner chase a suspect to a place called Fresnel Circle. Suddenly, the man splits into many different versions of himself, all going in expanded, concentric rings around the circle. What's going on? Only physics can explain.

Lenses are generally smooth things, with bulging middles tapering gently to thin sides. Contact lenses, telescope lenses, and the lenses in the eye are all smoothed out. The shape of lenses is determined by their intended function, which is generally to focus parallel streams of light to a point. The light is bent inwards by refraction when it enters the lens, and inwards again when it leaves the other side. The streams of light converge to a single point, which is where a person should position their eye (or their film) to get a single focused image.

The Fresnel Lens doesn't look or work like that. It's flat on one side, and chopped up into little shelf-like stairs on the other. The stairs are concenctric rings, slim towards the center and buidling up towards the edges. So a Fresnel lens looks a bit like a perfectly circular amphitheater standing on its side. The difference is the tops of each of the 'stairs' are cut at a slight angle. This angle bends the lights leaving the lens, forcing it to a focal point. It's at this focal point that someone would put their eye if they wanted a clear picture.

Although they wouldn't get nearly as clear a picture as they would if they had a smooth lens. Because of the spaces between the stairs, Fresnel lenses produce a wide beam of concentric rings instead of a focused single beam. They were first used because they cut out the bulk of a regular lens without giving up much of the transmission of light. They were used a great deal in early lighthouses, and in cheap magnifying glasses. The rotating lens in Fresnel Circle looks like many of the early lighthouse lenses, with many Fresnel lenses stuck back-to-back.