Let's talk about oil immersion.
We need to use oil to see at the highest magnification.
Without getting too technical there's an air
gap between the top of the coverslip
and the front lens.
This gap actually causes light to bend or
refract.
These scattered rays of light don't travel
through the objective lens and are lost to
the image.
Oil affects the light a lot like glass does,
so we replace the air gap with oil and suddenly
we're able to see higher resolution images.
