This is because of the refractive propoties of the immersion oil. The oil works as a sort of secondary lens, magnifying the specimin in addition to the lens alresdy present.
Oil reduces the effects of friction between the surfaces that are in contact with each other. If you have oil on the road it is not helpful as the moving car will slide around because the friction...
Immersion oil is only required when using a light manuscript at 1000 times magnification (10x from occular lens and 100x from objective lens). At 1000x magnification the image will have poor...
Refraction. Without the oil there, some of the light particles just bounce off the specimen and never bounce back into the lens for a fantastic image. With the oil there, all of the light is focused...
It is a procedure that is used to form a tiny oil "lens" on the objective of an optical microscope that yields high powers due to the higher refractive index of oil as compared to glass.