Oil immersion

In light microscopy, oil immersion is a technique used to increase the resolving power of a microscope.

Without oil, light waves reflect off the slide specimen through the glass cover slip, through the air, and into the microscope lens (see the colored figure to the right).

A measure of the resolving power, R.P., of a lens is given by its numerical aperture, NA: where λ is the wavelength of light.

From this it is clear that a good resolution (small δ) is connected with a high numerical aperture.

If the space between the objective lens and the specimen is filled with oil, however, the numerical aperture can obtain values greater than 1.

From the above it is understood that oil between the specimen and the objective lens improves the resolving power by a factor 1/n.

Objectives with high-power magnification have short focal lengths, facilitating the use of oil.

The correct immersion oil for an objective lens has to be used to ensure that the refractive indices match closely.

Cedar oil has a number of disadvantages: it absorbs blue and ultraviolet light, yellows with age, has sufficient acidity to potentially damage objectives with repeated use (by attacking the cement used to join lenses), and can change viscosity upon dilution with solvent (and thereby change its refraction index and dispersion).

[2] In modern microscopy, synthetic immersion oils are more commonly used, as they eliminate most of these problems.