When you order slides for your microscope, sometimes it comes with "Immersion oil"?

Question

What is this for and how do you use it with your slides?

Answer 1

The poster just above mine provided the correct reason for why it is used, but they failed to say when. On many microscopes today, there are 4 different settings; low (red), medium (yellow), high (blue), and oil immersion (usually represented with a white ring). For the low, medium and high settings, you do not need to use the oil. However, in order to see clearly on the highest, you would need to use the oil.

It might be opposite with the red and blue rings, I don't have a microscope in front of me at the moment.

Answer 2

Something called "numerical aperture" (often abbreviated N.A.) is what determines resolution and image quality. If you look on the side of an objective lens, the N.A. will be listed. Typically, numbers closer to 1 offer higher resolution. This will vary depending upon the type of lens being used. Long working-distance lenses, for example, have lower N.A.s. What the oil (or water immersion lenses) does it provide a medium between the objective lens system (because that is what an objective lens is, a system of lenses) and your sample with a higher refractive index than air. When the light exits the objective lens into the oil, it bends away from the normal, increasing the aperture of light which is illuminating the sample. (Specifically, it decreases interference of secondary order spots which lessen the resolution of your image). This gives you a higher resolution image. Immersion lenses are designed to work with the immersion medium (either oil or water) and they will give poorer resolution than a dry lens of the same magnifying power if you use them without the oil or water (i.e., in dry mode). In short, oil give you better resolution than using a 100x dry objective. I believe this is more critical in transmitted light microscopy. In practice, I find that water immersion lenses in reflected light microscopy work just fine. You can certainly use dry 100x objectives or 150x dry objectives. It is important to remember that higher magnification does not necessarily mean a better image though (which is why I never use the digital zoom on my personal camera).

Answer 3

The best optics for a microscope is with no air between the sample and the first lens. The oil has the same index of refraction as the lens on the microscope. So you cover the sample and the lens with the oil to get the best sharp image.
Lots of microscopes are to be used in air so you don't have to use the oil.

Answer 4

I have never heard of it, but I would think it would be for placing the objects on and then affixing the slide cover.