A single convex<unk>  mounted in a frame with a handle or stand is a magnifying glass.<unk> es are used as prosthetics for the correction of refractive errors such as myopia, hypermetropia, presbyopia, and astigmatism. (See corrective<unk> , contact<unk> , eyeglasses.) Most<unk> es used for other purposes have strict axial symmetry; eyeglass<unk> es are only approximately symmetric. They are usually shaped to fit in a roughly oval, not circular, frame; the optical centres are placed over the eyeballs; their curvature may not be axially symmetric to correct for astigmatism. Sunglasses'<unk> es are designed to attenuate<unk> ight; sunglass<unk> es that also correct visual impairments can be custom made. Other uses are in imaging systems such as monoculars, binoculars, telescopes, microscopes, cameras and projectors. Some of these instruments produce a virtual image when applied to the human eye; others produce a real image that can be captured on photographic film or an optical sensor, or can be viewed on a screen. In these devices<unk> es are sometimes paired up with curved mirrors to make a catadioptric system where the<unk> 's spherical aberration corrects the opposite aberration in the mirror (such as Schmidt and meniscus correctors). Convex<unk> es produce an image of an object at infinity at their focus; if the sun is imaged, much of the visible and infrared<unk> ight incident on the<unk>  is concentrated into the small image. A<unk> arge<unk>  creates enough intensity to burn a flammable object at the focal point. Since ignition can be achieved even with a poorly made<unk> ,<unk> es have been used as burning-glasses for at<unk> east 2400 years. A modern application is the use of relatively<unk> arge<unk> es to concentrate solar energy on relatively small photovoltaic cells, harvesting more energy without the need to use<unk> arger and more expensive cells. Radio astronomy and radar systems often use dielectric<unk> es, commonly called a<unk>  antenna to refract electromagnetic radiation into a collector antenna.<unk> es can become scratched and abraded. Abrasion-resistant coatings are available to help control this.
Lens
Eye
Globe 
Optician
DermAtlas
Pituitary gland
Heart
Optic stalk