Compound light microscopes use multiple lenses to view objects that are too small to be seen with the naked eye. These microscopes contain at least two lenses: an objective lens that is held near the object being viewed and an eyepiece--or ocular--lens that is positioned near the eye. Focal length is the most important characteristic of a lens and is related to how much the lens magnifies an object.
Microscope objectives are made of special optical glass that is of higher quality than the glass you find in most windows. The lens is shaped like a circular disk with the two faces curving outward, known as convex. When parallel rays of light strike one face of the objective lens, they are focused as they pass through and meet at a single spot called the focal point.
The distance from the center of the lens to the focal point is called the focal length. Because the image occurs on the other side of the lens from where the object is positioned, the focal length for convex lenses has a positive sign. Concave lenses--where the faces of the lens curve inward--have negative focal lengths.
Focal length is important because it determines the lens strength, which is an indication of how much the lens enlarges the image. Lens strength is calculated by dividing the number one by the focal length--taking the inverse of the focal length. A lens with a shorter focal length will have a higher lens strength and will enlarge the image more. Microscope objectives have short focal lengths to greatly enlarge the images.
The focal length of an objective is the distance from the lens to the point where parallel rays of light passing through the lens converge. The image created here then essentially becomes the object viewed by the ocular--or eyepiece--lens. When a larger image is created by an objective lens with a smaller focal length, the ocular lens views that larger image.