What is the function of the immersion oil?

Answer 1

Light, when it goes from a medium of a high refractive index to a medium of a lower refractive index is bent toward the medium having the higher refractive index. Therefore, light passing through a glass slide (refractive index of 1.52) into air (refractive index of 1.0) is reflected toward the slide instead of through the objective lens. This results in a fuzzy image and a loss of light. Immersion oil, with a refractive index comparable to glass, when placed between the slide and the oil immersion lens will correct this problem.