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`Concave Lens
`
`by Matt Williams on December 10, 2010
`
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`Concave & Convex Lens
`
`For centuries, human beings have been able to do some pretty remarkable things with lenses. Although we can’t be
`sure when or how the first person stumbled onto the concept, it is clear that at some point in the past, ancient people
`(probably from the Near East) realized that they could manipulate light using a shaped piece of glass. Over the
`centuries, how and for what purpose lenses were used began to increase, as people discovered that they could
`accomplish different things using differently shaped lenses. In addition to making distant objects appear nearer (i.e.
`the telescope), they could also be used to make small objects appear larger and blurry objects appear clear (i.e.
`magnifying glasses and corrective lenses). The lenses used to accomplish these tasks fall into two categories of
`simple lenses: Convex and Concave Lenses.
`
`A concave lens is a lens that possesses at least one surface that curves inwards. It is a diverging lens, meaning that it
`spreads out light rays that have been refracted through it. A concave lens is thinner at its centre than at its edges, and
`is used to correct short-sightedness (myopia). The writings of Pliny the Elder (23–79) makes mention of what is
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`http://www.universetoday.com/82338/concave-lens/
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`arguably the earliest use of a corrective lens. According to Pliny, Emperor Nero was said to watch gladiatorial games
`using an emerald, presumably concave shaped to correct for myopia.
`
`After light rays have passed through the lens, they appear to come from a point called the principal focus. This is the
`point onto which the collimated light that moves parallel to the axis of the lens is focused. The image formed by a
`concave lens is virtual, meaning that it will appear to be farther away than it actually is, and therefore smaller than
`the object itself. Curved mirrors often have this effect, which is why many (especially on cars) come with a warning:
`Objects in mirror are closer than they appear. The image will also be upright, meaning not inverted, as some curved
`reflective surfaces and lenses have been known to do.
`
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`The lens formula that is used to work out the position and nature of an image formed by a lens can be expressed as
`follows: 1/u + 1/v = 1/f, where u and v are the distances of the object and image from the lens, respectively, and f is
`the focal length of the lens.
`
`We have written many articles about concave lens for Universe Today. Here’s an article about the telescope mirror,
`and here’s an article about the astronomical telescope.
`
`If you’d like more info on the Concave Lens, check out NASA’s The Most Dreadful Weapon, and here’s a link to
`Build a Telescope Page.
`
`We’ve also recorded an entire episode of Astronomy Cast all about the Telescope. Listen here, Episode 150:
`Telescopes, The Next Level.
`
`Sources:
`http://en.wiktionary.org/wiki/concave
`http://www.physics.mun.ca/~jjerrett/lenses/concave.html
`http://encyclopedia.farlex.com/concave+lens
`http://en.wikipedia.org/wiki/Collimated_light
`http://en.wikipedia.org/wiki/Virtual_image
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