With the aid of a diagram outline the principle of oil immersion

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.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). Unless a wave comes out at a 90-degree angle, it twists when it hits a new substance, the amount of twist depending on the angle. This distorts the image. Air is very different from glass, making for a big twist compared to oil, which is more similar to glass. Ideally there would be glass all the way in between, but that would crack. Oil is the next best thing.

Immersion oils are transparent oils that have specific optical and viscosity characteristics necessary for use in microscopy. Typical oils used have an index of refraction around 1.515.[1] An oil immersion objective is an objective lens specially designed to be used in this way. Many condensers also give optimal resolution when the condenser lens is immersed in oil.