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In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope. This is achieved by immersing both the objective lens and the specimen in a transparent oil of high refractive index , thereby increasing the numerical aperture of the objective lens.
An oil immersion objective is an objective lens specially designed to be used in this way. The index of the oil is typically chosen to match the index of the microscope lens glass, and of the cover slip. For more details, see the main article, oil immersion. Some microscopes also use other index-matching materials besides oil; see water ...
Oil immersion is a technique that can increase image resolution by immersing the lens and the specimen in oil with a high refractive index. Since light bends when it passes between media with different refractive indexes, by placing oil with the same refractive index as glass between the lens and the slide, two transitions between refractive ...
In optical microscopes a darkfield condenser lens must be used, which directs a cone of light away from the objective lens. To maximize the scattered light-gathering power of the objective lens, oil immersion is used and the numerical aperture (NA) of the objective lens must be less than 1.0. Objective lenses with a higher NA can be used but ...
Oil immersion objectives typically have even larger numerical apertures so improved optical sectioning. The resolution in the depth direction (the "z resolution") of a standard wide field microscope depends on the numerical aperture and the wavelength of the light and can be approximated as:
Some microscopes use an oil-immersion or water-immersion lens, which can have magnification greater than 100, and numerical aperture greater than 1. These objectives are specially designed for use with refractive index matching oil or water, which must fill the gap between the front element and the object. These lenses give greater resolution ...
Some microscopes make use of oil-immersion objectives or water-immersion objectives for greater resolution at high magnification. These are used with index-matching material such as immersion oil or water and a matched cover slip between the objective lens and the sample. The refractive index of the index-matching material is higher than air ...
In a dry objective or condenser, this gives a maximum NA of 0.95. In a high-resolution oil immersion lens, the maximum NA is typically 1.45, when using immersion oil with a refractive index of 1.52. Due to these limitations, the resolution limit of a light microscope using visible light is about 200 nm.