Biomedical Engineering Reference
In-Depth Information
through the center of curvature, strikes the mirror perpendicular to its surface, and
is reflected back through the center of curvature (i.e., since the angle of incidence is
zero, the angle of reflection is also zero). In this case, where the object is farther away
than the center of curvature, the reflected rays intersect to form a real, inverted, and
minified image that is located to the left of the mirror. (As is the case with refracting
surfaces and lenses, the real images formed by mirrors are always inverted and can
be focused on a screen, while virtual images are always erect and cannot be focused
on a screen.)
A real image produced by a concave mirror is not always minified. As we can see
from Figure 14-3B, when the object is situated between the center of curvature and
the focal point, the inverted real image is larger than the object (and to the left of
the mirror).
In the two examples we just discussed, the object is located farther from the mir-
ror than the focal point. What happens when the object is located within the focal
length of the mirror? As is the case with a converging lens, an object located within
the focal length results in a virtual, erect, and magnified image (Fig. 14-3C). For a
mirror, the virtual image is to the right of its surface.
Convex Mirrors
Like diverging spherical refracting surfaces and lenses, convex mirrors have minus
power and diverge light. Figure 14-4A shows parallel light rays incident upon a
convex mirror. After reflection, these rays appear to emerge from the mirror's sec-
ondary focal point,
F
.
As can be seen in Figure 14-4B, the same four rays that are used to locate the
image formed by a concave mirror can be used to locate the image formed by a
convex mirror. A convex mirror forms a minified, erect, virtual image that is located
to the right of the mirror.
′
Plane Mirrors
A plane mirror is flat, meaning that it has an infinite radius of curvature. Unlike a
concave or convex mirror, it does not change the vergence of the light that is inci-
dent upon it and therefore has a power of zero.
Figure 14-5A shows an object that is located in front of a plane mirror. By apply-
ing the law of reflection to each of the light rays, we can locate the image. Note
that the image is virtual and located to the right of the mirror at the same distance
from the mirror as the object. If, for example, the object is 3.0 m in front of the
mirror, the virtual image is 3.0 m behind the mirror.
What is the magnification produced by a plane mirror? In Figure 14-5B, the
object is an arrow. As you can see, the separation of the top and bottom points of
the arrow is equal to the separation of the images of these two points. The image
produced by a plane mirror is the same size and orientation (erect) as the object
(i.e., the magnification is
+
1.0
×
).
