The condenser usually includes an iris that varies the amount of light passing through the specimen. After passing through the specimen, the light goes through the objective and ocular lenses, and then into the eye of the observer. The total magnification of the image is calculated by multiplying the magnification of the ocular by the magnification of the objective. This bending of light is called refraction. The refractive index of a substance is a measurement of the extent that the substance bends light.
Excessive refraction can cause distortion of the image.
At magnifications of less than 500x, the distortion is minimal. But at higher magnifications, the distortion becomes so great that image details are lost. An oil immersion lens helps to remedy this problem by eliminating the air gap between the specimen and the objective lens. A drop of special immersion oil is placed on the microscope slide, and the oil immersion objective lens is maneuvered so that it is touching the oil. Immersion oil has the same refractive index as glass so that the light passes through the slide, specimen, oil and objective lens as if they were a single piece of glass.