An optical microscope is made of a combination of two converging lenses, called the objective lens and the eyepiece or ocular lens. The purpose of such a microscope is to magnify small objects, and both lenses contribute to the final magnification. The magnification of a microscope decreases with increase in focal length of the objective. The length of tube affects the focal length of the lens used inside it; so, it affects the magnification of the microscope.