Perhaps the simplest optical instrument is the lens magnifier. Without optical aid, we cannot see things close up. The eye will simply not focus closer than about 0.25 m (unless you are nearsighted)
But an object places just inside the focal point of a converging lens will produce a large virtual image that can be viewed more easily. Let’s look at our Convex Lens again.
The standard optical microscope consists of two lenses (each can be a compound lens). By placing the object to be observed very close to the focal point of the first or objective lens, a larger real (but inverted) image will be produced. This real image is then observed with a second lens, the eyepiece, which acts as a magnifier to make the image even larger. If you wear eyeglasses or reading glasses, removing them will allow you to view the microscope properly.
Visit Here for more.