A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses arranged along a common axis. Lenses are made from materials such as glass or plastic, are ground and polished or molded to a desired shape. A lens can focus light to form an image, unlike a prism. Devices that focus or disperse waves and radiation other than visible light are called lenses, such as microwave lenses, electron lenses, acoustic lenses, or explosive lenses; the word lens comes from lēns, the Latin name of the lentil, because a double-convex lens is lentil-shaped. The lentil plant gives its name to a geometric figure; some scholars argue that the archeological evidence indicates that there was widespread use of lenses in antiquity, spanning several millennia. The so-called Nimrud lens is a rock crystal artifact dated to the 7th century BC which may or may not have been used as a magnifying glass, or a burning glass.

Others have suggested that certain Egyptian hieroglyphs depict "simple glass meniscal lenses". The oldest certain reference to the use of lenses is from Aristophanes' play The Clouds mentioning a burning-glass. Pliny the Elder confirms. Pliny has the earliest known reference to the use of a corrective lens when he mentions that Nero was said to watch the gladiatorial games using an emerald. Both Pliny and Seneca the Younger described the magnifying effect of a glass globe filled with water. Ptolemy wrote a book on Optics, which however survives only in the Latin translation of an incomplete and poor Arabic translation; the book was, received, by medieval scholars in the Islamic world, commented upon by Ibn Sahl, in turn improved upon by Alhazen. The Arabic translation of Ptolemy's Optics became available in Latin translation in the 12th century. Between the 11th and 13th century "reading stones" were invented; these were primitive plano-convex lenses made by cutting a glass sphere in half. The medieval rock crystal Visby lenses may not have been intended for use as burning glasses.

Spectacles were invented as an improvement of the "reading stones" of the high medieval period in Northern Italy in the second half of the 13th century. This was the start of the optical industry of grinding and polishing lenses for spectacles, first in Venice and Florence in the late 13th century, in the spectacle-making centres in both the Netherlands and Germany. Spectacle makers created improved types of lenses for the correction of vision based more on empirical knowledge gained from observing the effects of the lenses; the practical development and experimentation with lenses led to the invention of the compound optical microscope around 1595, the refracting telescope in 1608, both of which appeared in the spectacle-making centres in the Netherlands. With the invention of the telescope and microscope there was a great deal of experimentation with lens shapes in the 17th and early 18th centuries by those trying to correct chromatic errors seen in lenses. Opticians tried to construct lenses of varying forms of curvature, wrongly assuming errors arose from defects in the spherical figure of their surfaces.

Optical theory on refraction and experimentation was showing no single-element lens could bring all colours to a focus. This led to the invention of the compound achromatic lens by Chester Moore Hall in England in 1733, an invention claimed by fellow Englishman John Dollond in a 1758 patent. Most lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres; each surface can be concave, or planar. The line joining the centres of the spheres making up the lens surfaces is called the axis of the lens; the lens axis passes through the physical centre of the lens, because of the way they are manufactured. Lenses may ground after manufacturing to give them a different shape or size; the lens axis may not pass through the physical centre of the lens. Toric or sphero-cylindrical lenses have surfaces with two different radii of curvature in two orthogonal planes, they have a different focal power in different meridians. This forms an astigmatic lens. An example is eyeglass lenses. Lenses are classified by the curvature of the two optical surfaces.

A lens is biconvex. If both surfaces have the same radius of curvature, the lens is equiconvex. A lens with two concave surfaces is biconcave. If one of the surfaces is flat, the lens is plano-convex or plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is meniscus, it is this type of lens, most used in corrective lenses. If the lens is biconvex or plano-convex, a collimated beam of light passing through the lens converges to a spot behind the lens. In this case, the lens is called a converging lens. For a thin lens in air, the distance from the lens to the spot is the focal length of the lens, represented by f in diagrams and equations. An extended hemispherical lens is a special type of plano-convex lens, in which the lens's curved surface is a

Ghetto (Kelly Rowland song)

"Ghetto" is a song by American recording artist Kelly Rowland, featuring vocals by rapper Snoop Dogg. It was written by Durrell "Tank" Babbs, Calvin Broadus, Lonny Bereal and Rowland, produced by the former for Rowland's second solo album Ms. Kelly. A mid-paced R&B ballad, recorded for her shelved My Story album, "Ghetto" is influenced by the Crunk&B subgrene, its instrumentation consists of synthesizers and a drum machine rhythm and lyrically, finds Rowland, as the protagonist, singing sensually in her whistle register about becoming attracted to dangerous men. One of Rowland's favorites on Ms. Kelly, "Ghetto" was released as the album's second single to US radios on August 7, 2007, while "Work" was serviced as the album's second international single; the song performed weakly on the Billboard charts, reaching number nine on the Bubbling Under R&B/Hip-Hop Singles chart only, ranking it among Rowland's lowest-selling single to date. An accompanying music video, directed by Andrew Gura, was shot in Los Angeles, California in August 2007.

It feaures cameo appearances from rappers MC Eiht and Soopafly and depicts hip hop fashion and ghetto fabulous aesthetics, with Rowland and Snopp Doogg dancing. The song is one out of two recordings singer and songwriter Tank and partner Joseph "Lonny" Bereal produced for the album. One of her favorites on the album, Rowland has described "Ghetto" as "about good girls who dig bad boys," adding: "As soon as Tank played it, I knew and recorded it in an hour. Is just so laid back and that's why I had to have Snoop." Recorded without Snoop Dogg for the delayed 2006 My Story album, it was not until months prior to the Ms. Kelly release, that the rapper was consulted to contribute additional vocals to the track. "He's the King of Cool, so I was honoured when he said yes to do the record. It was like a dream come true," Rowland stated in an interview with Blue & Soul in 2007. "He put his Snoop-a-fied player-isms on the track. I like it when a song can make you feel good." "Ghetto" earned mixed reviews from contemporary music critics.

Sal Cinquemani from Slant felt that "Rowland displays her range on midtempo cut “Ghetto,” a slinky track featuring a smooth, sensual vocal from the singer." Spence D. of media website IGN observed that "the high pitched nuances continue on "Ghetto," featuring the ubiquitous Snoop Dogg. Rowland adopts a wispy timbre, sensual in a fleeting way, but the track is again amped up with too much high end sound, making it more cloying than immersive." San Francisco Chronicle remarked that "Ghetto" has "Rowland exploring the common theme of a good girl falling for a bad boy. Her cotton-candy mezzo tones are easy to digest." Mark Edward Nero from found that "Ghetto" was the worst of the "many weak songs" on Ms. Kelly, concluding that "it's laugh-out-loud ridiculous with Kelly singing contrived, out-of-character lines." CBS News called the song "sexy and seductive" Released in North America only, "Ghetto" went for radio ads in August 2007 and was added by about twenty-eight urban contemporary stations and few rhythmic contemporary stations.

On September 1, 2007, it debuted on the US Billboard Bubbling Under R&B/Hip-Hop Singles chart at number nine, equivalent to number 109 on the Hot R&B/Hip-Hop Songs chart. As it neither managed to climb any higher on that particular chart, nor made it to any other official chart, the songs ranks as Rowland's lowest-selling single to date. Although Rowland noted that it was her decision to pick the track as the album's second single, she somewhat regretted to release it retrospectively. An accompanying music video for "Ghetto" was directed by Andrew Gura and filmed in Los Angeles, California on August 23, 2007. Next to Rowland and Snoop Dogg, rappers MC Eiht and Soopafly make cameo appearances in the visual. Production on the video was helmed by Partizan Entertainment, with Melissa Larsen serving as producer. Main camera was operated by Shawn Kim. Special effect were provided by Laundy. Displaying hip hop fashion and ghetto fabulous aesthetics, the video depicts Rowland and Snoop Dogg dancing and performing in front of different lowriders, intercut by animated solo performances of Rowland.

When asked about the conception of the video, Rowland elaborated: "It's ghetto. It's fly. I feel. Think about it. Right? Wouldn't you say so? And everyone knows what the ghetto is."The video world premiered at the end of the September 10, 2007 episode of BET's televsion series Access Granted. In his review for Videostatic, editor Craig Belcher wrote: "Rowland gets hoodcentric with this tribute to the rough life with Snoop Dogg as the guest rapper and the object of her lyrical affections. If she was looking to shed her good-girl image this video puts her in a different light. Rowland gyrates as dreamy images of street culture drift behind her, including 24 inch rims, platinum jewelry, golden grills All that's missing is a bag of sunflower seeds and strawberry soda." Credits adapted from the liner notes of Ms. Kelly. "Ghetto" music video on YouTube Lyrics of this song at MetroLyrics

Multi-directional Impact Protection System

Multi-directional Impact Protection System, MIPS, is a technology that allows a helmet slide relative to the head, adding more protection against rotational violence to the brain caused by angled impacts. It was developed by biomechanical specialists at the Royal Institute of Technology, in Stockholm, Sweden. MIPS is a form of slip plane technology, meaning that a MIPS helmet is constructed from two layers that rotate against each other, mimicking the rotation of the brain’s own cerebrospinal fluid, the body’s natural defense against oblique impacts; the outer layer of a MIPS helmet is made from the same impact-absorbing EPS material as a conventional helmet. It’s connected via an elastomeric attachment system to a low friction inner layer, what rests on the rider’s head. In a crash, the outer shell of a MIPS helmet absorbs linear impact, while the inner layer rotates up to 5 mm, absorbing rotational impact; this small rotation of the liner relative to the shell results in a significant reduction of the forces on the brain, which reduces the likelihood of concussion and other brain injury.

In 1995 Swedish neurosurgeon Hans von Holst began evaluating the general construction of helmets. Following several years of study and work related to head trauma for the World Health Organization in Geneva, he determined existing helmet technology provided inadequate protection against brain trauma – with devastating consequences for victims of brain injuries. Together with Royal Institute of Technology researcher Peter Halldin, von Holst developed a technology that would provide a more effective protection against head trauma. After evaluating the brain anatomy and combining their years of testing and expertise, von Holst and Halldin produced what is now known as the Multi-directional Impact Protection System, a technology that mimics the brain’s own protective structure; the company MIPS AB was founded in 2001 by five specialists in biomechanics of the Royal Institute of Technology in Stockholm, Sweden. Following its inception in 2001, MIPS AB evolved from concept to product, introducing the technology in an equestrian helmet 2007.

As MIPS made significant expansions internationally, its unique branding was shaped in 2009 giving the Stockholm-based company a clear and distinct identity in logo and all online platforms. Beginning in 2010, MIPS AB took on the bike and snow helmet industry, gaining widespread recognition and the technology was adopted by motocross, mountain biking, skiing and other sports industries where head trauma is common; as the awareness level of head trauma injuries has increased, so, have the awareness and benefits of technology such as MIPS. In 2013, Peter Halldin and Daniel Lanner from MIPS AB, Richard Coomber from Revision Military, Montreal Canada, Sven Kleiven from KTH published a paper describing both a numerical and an experimental approach to measuring the blunt-impact protection offered by military helmets. In early 2014, one of the world’s largest helmet manufacturers, BRG Sports, Inc and MIPS AB entered into a formal partnership after several years of sharing their technology, offering MIPS technology to a much wider global audience.

Since MIPS AB has added several new brands to its catalog and has expanded to the American and Chinese markets. In the end of 2015, MIPS produced its millionth layer. Official website