The objective is the first lens that light from the spcimen passes through, and is made to have a short focal length. The final lens is the eye piece, which further magnifies the image, and creates an image perceivable by the human eye. The optical or light microscope uses visible light transmitted through, refracted around, or reflected from a specimen.
The aperture of a microscope determines its resolving power. Resolution is the ability of a microscope to keep all specific details of the object intact in the image.
Hence, all microscopes, based on the technique being used, have a magnification limit at which there is no longer more resolution.
Light waves are chaotic; an incandescent light source emits light waves traveling in different paths and of varying wavelengths. Some of the lenses in a microscope bend these light waves into parallel paths, magnify and focus the light at the ocular. Simple ways to get around this is through techniques like oil immersion, which create a refractive index difference to bend the light inwards, and increase the magnification limit.
Magnification is expressed in numeric multiples of how much enlargement occurs with a lens. If the magnification of a lens is 2X then it roughly doubles the size of the image of the object.