In: Physics
Considering the illustrations for Concave and Convex mirrors.
Prove using geometry that the reflected rays reach the focal point f=R/2 in the limit as the incoming rays approach the principal axis.
Hint: Consider the triangle formed by the radius of curvature, principal axis, and reflected ray, and use the law of sines.
There are two alternative methods of locating the image formed by a concave mirror. The first is purely graphical, and the second uses simple algebraic analysis.
The graphical method of locating the image produced by a concave mirror consists of drawing light-rays emanating from key points on the object, and finding where these rays are brought to a focus by the mirror. This task can be accomplished using just foursimple rules:
The validity of these rules in the paraxial approximation is fairly self-evident.
Consider an object which is placed a distance
from a concave spherical mirror,
as shown in Fig. 71. For the sake of definiteness, let us suppose
that the object distance
is greater than the focal length
of the mirror. Each point on the
object is assumed to radiate light-rays in all directions. Consider
four light-rays emanating from the tip
of the object which strike the
mirror, as shown in the figure. The reflected rays are constructed
using rules 1-4 above, and the rays are labelled accordingly. It
can be seen that the reflected rays all come together at some point
. Thus,
is the image of
(i.e., if we were to
place a small projection screen at
then we would see an image of
the tip on the screen). As is easily demonstrated, rays emanating
from other parts of the object are brought into focus in the
vicinity of
such that a complete image of
the object is produced between
and
(obviously, point
is the image of point
). This image could be viewed by
projecting it onto a screen placed between points
and
. Such an image is termed a
real image. Note that the image
would also be directly visible
to an observer looking straight at the mirror from a distance
greater than the image distance
(since the observer's eyes could
not tell that the light-rays diverging from the image were in
anyway different from those which would emanate from a real
object). According to the figure, the image is inverted
with respect to the object, and is also magnified.
Figure 72 shows what happens when the object distance is less than the focal length
. In this case, the image appears
to an observer looking straight at the mirror to be located
behind the mirror. For instance, rays emanating from the
tip
of the object appear, after
reflection from the mirror, to come from a point
which is behind the mirror. Note
that only two rays are used to locate
, for the sake of clarity. In
fact, two is the minimum number of rays needed to locate a
point image. Of course, the image behind the mirror cannot be
viewed by projecting it onto a screen, because there are no real
light-rays behind the mirror. This type of image is termed a
virtual image. The characteristic difference between a
real image and a virtual image is that, immediately after
reflection from the mirror, light-rays emitted by the object
converge on a real image, but diverge from a
virtual image. According to Fig. 72, the image is upright
with respect to the object, and is also magnified.
The graphical method described above is fine for developing an intuitive understanding of image formation by concave mirrors, or for checking a calculation, but is a bit too cumbersome for everyday use. The analytic method described below is far more flexible.
Consider an object placed a distance
in front of a concave mirror of
radius of curvature
. In order to find the image
produced by the mirror, we draw
two rays from
to the mirror--see Fig. 73. The
first, labelled 1, travels from
to the vertex
and is reflected such that its
angle of incidence
equals its angle of reflection.
The second ray, labelled 2, passes through the centre of curvature
of the mirror, strikes the
mirror at point
, and is reflected back along its
own path. The two rays meet at point
. Thus,
is the image of
, since point
must lie on the principal
axis.
In the triangle , we have
, and in the triangle
we have
, where
is the object distance, and
is the image distance. Here,
is the height of the object, and
is the height of the image. By
convention,
is a negative number, since the
image is inverted (if the image were upright then
would be a positive number). It
follows that
![]() |
(351) |
Thus, the magnification of the image with respect to the
object is given by
![]() |
(352) |
By convention, is negative if the image is
inverted with respect to the object, and positive if the image is
upright. It is clear that the magnification of the image is just
determined by the ratio of the image and object distances from the
vertex.
From triangles and
, we have
and
, respectively. These
expressions yield
![]() |
(353) |
Equations (352) and (353) can be combined to give
![]() |
(354) |
which easily reduces to
![]() |
(355) |
This expression relates the object distance, the image distance, and the radius of curvature of the mirror.
For an object which is very far away from the mirror
(i.e., ), so that light-rays from the
object are parallel to the principal axis, we expect the image to
form at the focal point
of the mirror. Thus, in this
case,
, where
is the focal length of the
mirror, and Eq. (355) reduces to
![]() |
(356) |
The above expression yields
![]() |
(357) |