What is the function of using oil under 100x objective?

In physics, you learn about the refraction, or bending of light. Under a 100x objective lens, light will refract in different ways, causing the image to look very unfocused. However, oil has an index of refraction similar to the gl lens in the objective. If you look closely, you will actually see that the lens touches the oil on the slide, hence the name "Oil immersion lens". Because these two indicies are very similar, light does not refract as much, enabling you to focus the image and see the image at a very high magnification.