Chapter 49
Optical Instruments
In this chapter, we’ll examine a number of common optical instruments, both natural and man-made. These
instruments are designed to form and record simple images (the eye and the camera), to make enlarged images
of very small, close objects (the magnifying glass and microscope) or of very distant objects (the telescope),
or to simply shift an image through some distance (the periscope).
49.1 The Magnifying Glass
In its simplest form, amagnifying glassormagnifierconsists of a single converging (convex) lens. The
human eye can normally get as close as about 25 cm from an object and still have it comfortable in focus;
this is called thenear point. By placing a magnifying glass near the eye (so the eye is closer to the lens than
the focus), an enlarged virtual image of the object is created. (See Figure 49.1.)
High-power magnifying glasses often contain a compound lens, consisting of two or more single lenses
cemented together. The combination of lenses can help correct unwanted optical defects.
49.2 The Human Eye
Thehuman eyeis a naturally occurring optical instrument that gives humans their sense of sight. The active
optical components are thecorneaand thelens. Most of the focusing of the image is done by the cornea,
while the lens acts as a secondary optical element. The image produced by the cornea and lens is focused onto
theretinaon the back of the eye. (Figure 49.2.) The image projected onto the retina is actually upside-down;
our brains invert the image so that we seem to see the image rightside-up.
The retina is covered with a grid of two kinds of light detectors:rodscan detect very faint light, but
produces only black-and-white images.Conesrequire a somewhat brighter light level before they activate,
but they can see in color. There are three types of cones: one type is most sensitive to red light, another
most sensitive to green light, and another most sensitive to blue light. The brain receives signals of different
strengths from each type of cone at each location in the image, and from that is able to infer the color of that
part of the image. (Any color can be formed from combinations of the three primary colors red, green, and
blue; see chapter 56 on color.) From the retina, signals are transmitted to the brain via theoptic nerve.
Things are a bit more complicated than this, though. The eye and brain are able to perceive objects as
having a constant color, even when viewed under widely varying lighting conditions. It is believed that the eye
views an entire image (by means of cones) in each of the three primary colors, then sends this information to
the brain; the brain then determines the correct color both from the strength of the signals from the different
colors of cones, and also by comparing the perceived brightness of each part of the image with those of