In this chapter we describe the basic principles of some of the commonly used optical instruments. We start with the most common, the human eye, and discuss how spectacles correct near- or farsightedness. We then discuss a magnifier (or a reading glass), a microscope, and a telescope. We illustrate how the eye interacts with such instruments when images are observed by humans. A pinhole camera is also described briefly.
The human eye is a visual positive lens system that forms a real image on the retina, as illustrated in Figure 6-1. It is nearly spherical, with a diameter of about 2.5 cm among adults and a tough 1-mm-thick outer shell called the sclera. Its front portion, where the eye bulges outward, represents the first element of the lens system called the cornea. It is a transparent tissue approximately 0.5 mm thick, with a refractive index of 1.377, while the rest of the sclera is white and opaque. Nearly two-thirds of the bending of object rays takes place at the air–cornea interface. The cornea is also slightly reflective and acts like a convex mirror, resulting in our ability to see ourselves in the eyes of another person. Because the refractive index of the cornea is very close to that of water (1.333), no significant refraction takes place at a water–cornea interface. Accordingly, a person cannot see very well under water (divers wear a mask that creates airspace between the water and the eye). The eyelids protect the delicate cornea from foreign particles. By blinking constantly, they keep a layer of tears on the cornea. The tear film is produced by glands within the lids. Without the tears, a dry cornea loses its transparency.
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