Light and optics: Analysis of image production by the human eye

While optical instruments can analyze the properties of light, such as interference and refraction with the interferometer and refractometer, respectively, a more familiar role of optical instruments is for image enhancement. There are two types of lens, convex and concave, but converging lens are much more prevalent since they can function as an image producer and magnifier.

The cornea and lens form a system that acts approximately like a single thin lens. Nonetheless, refraction primarily occurs at the cornea and then secondarily at the lens, so most of the power of the eye can be attributed to the cornea. For clear vision, a real image must be projected precisely onto the retina. Because the lens-to-retina distance does not change, the image distance remains the same at 2.0 cm for objects at all distances. The eye manages around this restriction by varying the power (and focal length) of the lens for objects at various distances, i.e. accommodation. It allows a person with normal vision to see objects clearly at distances ranging from 25 cm to essentially infinity.

In the laboratory, the most common instrument for image enhancement is the compound microscope, which is the archetype of a multiple lens system. The first lens is called the objective lens and has typical magnification values from 5× to 100×. In standard microscopes, they are mounted such that when you switch between objectives the sample remains in focus. The second, called the eyepiece or ocular, has several lenses which slide inside a cylindrical barrel, but the lenses in the eyepiece have been simplified to a single lens in Figure 2. The focusing ability is provided by the movement of both the objective lens and the eyepiece. A final consideration is that the final image be produced in a location far enough to be easily viewed, since the eye cannot focus on images that are too close.