oil-immersion lens

An oil-immersion lens is a type of high-power microscope objective that requires a drop of oil between the specimen slide and the lens. This oil has a refractive index that matches the glass of the objective's first component. This technique is used to increase the numerical aperture (N.A.) of the objective, which in turn improves the microscope's resolution capabilities. The N.A. is a measure of the lens's ability to gather light and resolve detail, and it is influenced by the refractive index of the medium between the object and the lens. By using oil, the effective wavelength of light is reduced, allowing for finer details to be distinguished.12