An oil immersion objective is designed to be used with oil between the front lens and the cover glass, thereby making full use of the NA of the objective.
AN is possibly the most important criterion when selecting a target. As NA is increased across series of objectives of the same magnification, light collection capacity and resolution increase. Unlike air, immersion oil has a refractive index similar to that of cover slip glass. By placing immersion oil between the front objective lens (designed for use with immersion oil) and the cover glass, the angular range of the diffracted rays captured by the lens is increased and, in effect, increases the NA of the objective. Light rays passing through the specimen, covering objects, and oil are not refracted as they enter the lens but only as they leave the top surface. Objectives that use water and/or glycerin as the imaging medium are also available.
Different microscope manufacturers produce objectives with close tolerances for refractive and dispersion indices and require matching values in the oil (or other media) placed between the cover glass and the front objective lenses. Users should therefore only use the oil (or other media) recommended by the manufacturer.
The higher resolution gained through the use of immersion oil enables users to focus on very small objects that would not be resolved using dry objectives. Oil immersion objectives are used for observation of very small targets, such as individual bacteria, and high resolution applications, such as TIRF and confocal fluorescence applications.
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