Why do we use oil with microscopes?

The use of oil with microscopes, specifically with oil immersion lenses, significantly increases the amount of light that enters the objective lens. This is because oil has a refractive index that is closer to that of glass than air does. When using an immersion oil, the light rays that pass through the specimen and into the lens do not undergo as much refraction as they do when passing through air, which can often lead to loss of detail and brightness. By minimizing the gap between the specimen and the lens, oil enhances the clarity and brightness of the images, allowing finer details of the specimen to be observed. This technique is particularly important when examining specimens at high magnification, where the resolution and detail are critical for accurate observation and analysis.