Objective (optics): Difference between revisions
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In an [[optics|optical]] instrument, the '''objective''' is the optical element that gathers light from the object being observed and focuses the [[ray (optics)|light rays]] to produce a [[real image]]. Objectives can be single [[Lens (optics)|lenses]] or [[mirror]]s, or combinations of several optical elements. They are used in [[microscope]]s, [[telescope]]s, [[camera]]s, [[slide projector]]s, [[CD player]]s and many other optical instruments. Objectives are also called '''object lenses''', '''object glasses''', or '''objective glasses'''. |
In an [[optics|optical]] instrument, the '''objective''' is the optical element that gathers light from the object being observed and focuses the [[ray (optics)|light rays]] to produce a [[real image]]. Objectives can be single [[Lens (optics)|lenses]] or [[mirror]]s, or combinations of several optical elements. They are used in [[microscope]]s, [[telescope]]s, [[camera]]s, [[slide projector]]s, [[CD player]]s and many other optical instruments. Objectives are also called '''object lenses''', '''object glasses''', or '''objective glasses'''. |
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==Types== |
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Microscope objectives are typically designed to be [[Parfocal lens|parfocal]], which means that when one changes from one lens to another on a microscope, the sample stays in [[focus (optics)|focus]]. Microscope objectives are characterized by two parameters, namely, [[magnification]] and [[numerical aperture]]. The former typically ranges from 5× to 100× while the latter ranges from 0.14 to 0.7, corresponding to [[focal length]]s of about 40 to 2 mm, respectively. For high magnification applications, an [[oil-immersion objective]] or water-immersion objective has to be used. The objective is specially designed and [[Index-matching material|refractive index matching oil]] or water must fill the air gap between the front element and the object to allow the [[numerical aperture]] to exceed 1, and hence give greater resolution at high magnification. Numerical apertures as high as 1.6 can be achieved with oil immersion.<ref>{{cite web |url=http://www.olympusmicro.com/primer/anatomy/objectives.html |title=Microscope objectives |work=Olympus Microscopy Resource Center |first=Spring |last=Kenneth |coauthors=Keller, H. Ernst; Davidson, Michael W. |accessdate=29 Oct 2008}}</ref> |
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The objective lens of a Microscope is the one at the bottom near the sample. At its simplest it is a very high powered magnifying glass i.e. a lens with a very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus about 160 mm inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample. Typically there is more than one objective lens of different focal lengths screwed into a circular nose piece which may be rotated to select the required lens. |
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A telescope's light gathering power and [[angular resolution]] is directly related to the diameter (or aperture) of its objective lens or mirror. The larger the objective, the dimmer the the object it can view and the more detail it can resolve. |
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To find the total magnification of a microscope, one multiplies the magnification of the objective lenses by that of the [[eyepiece]]. |
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Revision as of 22:09, 14 January 2011
In an optical instrument, the objective is the optical element that gathers light from the object being observed and focuses the light rays to produce a real image. Objectives can be single lenses or mirrors, or combinations of several optical elements. They are used in microscopes, telescopes, cameras, slide projectors, CD players and many other optical instruments. Objectives are also called object lenses, object glasses, or objective glasses.
Types
The objective lens of a Microscope is the one at the bottom near the sample. At its simplest it is a very high powered magnifying glass i.e. a lens with a very short focal length. This is brought very close to the specimen being examined so that the light from the specimen comes to a focus about 160 mm inside the microscope tube. The objective itself is usually a cylinder containing one or more lenses that are typically made of glass; its function is to collect light from the sample. Typically there is more than one objective lens of different focal lengths screwed into a circular nose piece which may be rotated to select the required lens.
A telescope's light gathering power and angular resolution is directly related to the diameter (or aperture) of its objective lens or mirror. The larger the objective, the dimmer the the object it can view and the more detail it can resolve.
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A photographic objective, focal length 50 mm, aperture 1:1.4 -
Diastar projection objective from a 35 mm movie projector, (focal length 400 mm). -
Two Leica oil immersion microscope objective lenses; left 100x, right 40x. -
The segmented hexagonal objective mirror of the Keck 2 Telescope
References
See also
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