Light microscopy
Light microscopy is the most commonly used microscopy technique. It often requires staining of the specimen to be able to visualize the structures of interest.
Light microscopy is limited to a minimal resolution of approximately 200 nanometers (nm). The minimal resolution is defined as the distance between two points that are still distinguishable as two separate entities. The resolution is limited by physical properties of light and the lens of the microscope. On one hand, the wavelength of the light limits the resolution; the shorter the wavelength the better the resolution. On the other hand, the aperture value of the objective lens limits the resolution.
To tune the aperture value to its limits you can place a drop of immersion oil between the coverslip of the slide and the objective lens. Without the immersion oil, the light is refracted when it moves from glass to air and back into the glass of the objective lens. If you use immersion oil with the same optical density as glass, this effect can be diminished and the minimal resolution can be achieved.