Figure 49.6: A simple camera. (Ref. [4])49.4 The Camera
Acamerais an instrument used to record an optical image. It is similar in design to the human eye: the image
of an object is focused by a lens onto a plane, where the image is recorded (Fig. 49.6). At one time, images
were recorded on chemically-coated glass plates; later, a flexible plastic chemical-coatedfilmwas used. Since
the 1990s, it has become very common to replace the film with a CCD (charge-coupled device) detector that
can record a digital image.
A very simple type of camera is called apinhole camera, in which the lens is replaced by a very small
hole. One could build a very simple, inexpensive camera by placing (in a darkened room!) photographic
film at one end of a lightproof box (a shoebox, for example) that has a covered pinhole at the other end. To
take a picture, the pinhole is uncovered for several seconds; the film is then removed in a darkened room and
developed(chemically processed to bring out the image).
If the distance from the pinhole to the film isLand the wavelength of light is , the it can be shown that
the optimum diameterdof the pinhole is given by
dDp
2L : (49.1)Most modern cameras use a lens instead of a pinhole, and many have a variety of settings to control
the focus and aperture size. Focusing is accomplished by changing the distance between the lens and the
film plane: the closer the object, the farther the lens must be from the film. This is because of the relation
1=diC1=doD1=f: decreasing the object distancedocauses1=doto increase; but sincefis constant, that
means1=dimust decrease to compensate, which means the image distancedimust increase. To take extreme
closeups, some cameras can be equipped with a set ofextension ringsthat allow the lens to be placed very far
away from the film. This allows one to take photographs of objects like grains of salt, as if they were being
seen under a microscope.
49.5 The Microscope
Amicroscope(from the Greek
o&, “small”, and
o!, “see”) is an instrument that allows one to
seeverysmall objects—generally much smaller than can be seen with a magnifying glass. (See Fig. 49.7.)
The optics consist of a short-focal lengthobjective lensthat is placed near the object, and aneyepiecethat