Why do we use oil to see bacterios in 100X magnification in compound microscope ?

Answer 1

Generally an oil immersion objective is used with magnifications greater than around 400X. At very high magnifications a regular
objective lens does not provide a good resolution (clarity).
A drop of immersion oil between the objective lens and specimen
allows for better resolution at high magnification. The reason for this can be explained by someone familiar with physics and
the properties of light.