The object (or specimen) being imaged by the lens is positioned in the object plane, located on the left-hand side of the lens by convention, and is represented by a red arrow that travels upward from the centerline or optical axis, which passes through the center of the lens, perpendicular to the principal planes. Because the bi-convex lens illustrated in Figure 2 is symmetrical, the principal planes are located equal distances from the lens surfaces, and the front and rear focal lengths are also equal. The principal planes of the lens ( P and P' in Figure 2) are denoted by dashed lines, and the distance between each principal plane and its respective focal point represents the focal length ( f). The focal points of the lens are denoted by the variable F, and there are two separate focal points, one in front of the lens (on the left-hand side of Figure 2) and one behind the lens (on the right). Traces of light rays passing through a simple bi-convex thin glass lens are presented in Figure 2, along with the other important geometric parameters necessary in forming a focused image by the rays. The focal length of a lens is defined as the distance between the principal plane and the focal plane, and every lens has a set of these planes on each side (front and rear). Light rays passing through the lens will intersect and are physically united at the focal plane (see Figure 2), while extensions of the rays entering the lens will intersect at the principal plane with extensions of the rays emerging from the lens. Each lens has two principal planes and two focal planes that are defined by the geometry of the lens and the relationship between the lens and the focused image. In order to understand the optical system of a simple microscope (locations of the lens elements in a common laboratory microscope are presented in Figure 1), the basic properties of a simple thin lens having two light-refracting surfaces and a central optical axis must first be described. The basic concepts explored in this discussion, which are derived from the science of Geometrical Optics, will lead to an understanding of the magnification process, the properties of real and virtual images, and lens aberrations or defects.
The action of a simple lens, similar to many of those used in the microscope, is governed by the principles of refraction and reflection and can be understood with the aid of a few simple rules about the geometry involved in tracing light rays through the lens.
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