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Lens scope is basically determined by the objec- Eyepiece
Pupil tive lens, which is placed close to the speci- lens
Object
Retina Fovea men on the stage of the microscope. Light
is projected up through the specimen, and Focus
the objective lens makes an enlarged image knob
Optic of the specimen inside the tube between the
Cornea nerve two lenses. The eyepiece lens is adjusted up Objective
lens
Iris and down to make a sharp enlarged image
Vitreous humor Glass
of the image produced by the objective lens slide
BOX FIGURE 7.3 Light rays from a (Box Figure 7.5). Object
distant object are focused by the lens onto the Telescopes are optical instruments Stage
retina, a small area on the back of the eye.
used to provide enlarged images of near
and distant objects. There are two major
types of telescope: refracting telescopes that Mirror
The microscope is an optical device use two lenses and reflecting telescopes that
used to make things look larger. It is essen- use combinations of mirrors, or a mirror
tially a system of two lenses, one to produce and a lens. The refracting telescope has
BOX FIGURE 7.5 A simple microscope
an image of the object being studied, and two lenses, with the objective lens form-
uses a system of two lenses, which are an
the other to act as a magnifying glass and ing a reduced image, which is viewed with
objective lens that makes an enlarged image
enlarge that image. The power of the micro- an eyepiece lens to enlarge that image.
of the specimen and an eyepiece lens that
makes an enlarged image of that image.

In reflecting telescopes, mirrors are used
instead of lenses to collect the light (Box
Figure 7.6).
Finally, the digital camera is a more
recently developed light-gathering and
photograph-taking optical instrument.
This camera has a group of small photo-
A Normal vision, distant object B Normal vision, near object
cells, perhaps thousands, lined up on the
focal plane behind a converging lens. An
image falls on the array, and each photo-
cell stores a charge that is proportional to
the amount of light falling on the cell. A
microprocessor measures the amount of
charge registered by each photocell and
considers it as a pixel, a small bit of the
overall image. A shade of gray or a color
C Nearsighted, uncorrected D Nearsighted, corrected is assigned to each pixel, and the image
is ready to be enhanced, transmitted to a
screen, printed, or magnetically stored for
later use.

Incoming light Reflecting mirror Concave
mirror

E Farsighted, uncorrected F Farsighted, corrected Eyepiece
lens
BOX FIGURE 7.4 (A) The relaxed, normal eye forms an image of distant objects on the BOX FIGURE 7.6 This illustrates how
retina. (B) For close objects, the lens of the normal eye changes shape to focus the image on the path of light moves through a simple
the retina. (C) In a nearsighted eye, the image of a distant object forms in front of the retina. reflecting astronomical telescope. Several
(D) A diverging lens corrects for nearsightedness. (E) In a farsighted eye, the image of a different designs and mirror placements are
nearby object forms beyond the retina. (F) A converging lens corrects for farsightedness. possible.

7-11 CHAPTER 7 Light 187