Microscope immersion oil is used in light microscopy to improve imaging. The use of microscope immersion oil as part of a microscope lens system will produce a brighter and sharper image than a similar design not using immersion oil.
Oil Immersion Microscopy increases the refractive index of a specimen when used properly. With limited disadvantages, slides prepared with oil immersion techniques work best under higher magnification where oils increase refraction despite short focal lengths.