What Are Flat & Curved Lenses?

Flat and curved lenses are used to focus light to produce an image. Curved or flat, each lens has a front and back surface. To be defined as a lens, a lens must have at least one surface that is curved. A flat -- also known as plano -- lens has one curved surface and one flat surface. A curved lens is crescent-shaped and has two curved surfaces. If it is thicker in the middle and thinner at the edges, it is a convex or positive lens. If it is thinner in the middle and thicker at the edges it is a concave or negative lens. A convex surface on a mirror or lens magnifies an image; a concave surface minimizes it. When a viewing device combines multiple lenses, it is called a compound lens.

Flat lenses

• Old-fashioned spectacles were made with flat lenses that required the wearer to look straight ahead to see objects in focus.

  Flat or plano lenses were the only option from the Middle Ages to the 19th century, when the equipment to grind lenses was hand- or foot-powered. By necessity, the lens had to be kept simple because the technology to produce it was simple. During those centuries, spectacles were pieces of glass that were simple ground lenses, curved on one surface and flat on the other. Flat-lens spectacles require the wearer to look straight through the center of the lenses to keep objects in focus. Moving the eyes' line of sight up or down from the middle of a flat lens causes distortions in what the eyes see. Flat lenses are used today in the Fresnel lenses used in lighthouse lamps and in headlights, stoplights and flat plastic magnifying sheets that are used as reading aids for those with poor eyesight.

Concave Lenses

• A peep hole is an example of a concave lens.

  Concave lenses are made of glass or molded from plastic to have a surface that curves inward. This inward curve refracts light outward, making it diverge or widen. Less common than convex lenses, concave lenses are used in big-screen projection televisions, door peep holes and the concave mirrors that are used to watch customers in stores to deter shoplifting. They produce the opposite effect of magnification, minimizing objects to make them appear further away.

Convex Lenses

• The convex lens of a telescope makes far-away objects appear closer.

  Convex lenses are ground or formed to curve or bulge out, producing a lens that is thick in the middle and thinner at its edges. A convex lens causes light rays to narrow, or converge, and makes objects viewed through them appear closer than they are. A telescope is made of two convex lenses, one at each end of a tube. In 1610, the Italian scientist and philosopher Galileo Galilee made a telescope equipped with convex lenses that allowed him to observe the Moon's surface and the rings of Saturn.

Compound Lenses

• Compound microscopes are equipped with compound, or multiple, lenses.

  Compound lenses combine multiple positive (outward curving) convex and negative (inward curving) concave lenses to view an image more clearly using one device. Compound microscopes and bifocal eyeglasses are examples of compound lenses.