Answers to Even-numbered Conceptual Questions
Three images are formed of object B.
One extends from (–3 m, 1 m) to (–3 m, 2 m) to
(–4 m, 2 m).
Another image forms an “L” from (3 m, –1 m) to (3 m, –2 m) to (4 m, –2
Finally, the third image extends from (–3 m, –1 m) to (–3 m, –2 m) to (–4 m, –2 m).
As can be seen in the photo accompanying this question, the hands on the mirror-image
clock rotate counterclockwise.
The main mirror in a telescope is always concave, because concave mirror focus parallel
rays of light (as from the stars) to a point in front of the mirror.
Convex mirrors, on the
other hand, disperse parallel rays of light by sending them outward on divergent paths.
Looking at the front side of a spoon means we are looking at a concave mirror.
addition, holding the spoon at arm’s length means that we are outside the focal point of
the mirror – clearly, the focal length of the front side of a spoon is only a few centimeters.
The situation, then, is like that illustrated in Figure 26-18 (a).
It follows that our image is
reduced, real, and inverted.
Referring to Figure 26-18 (a), we see that as the object is moved farther to the left, the
image moves to the right – toward the focal point of the mirror.
The concave side of the dish collects the parallel rays coming from a geosynchronous
satellite and focuses them at the focal point of the dish.
The convex side of the dish would
send the parallel rays outward on divergent paths.
The situation is analogous to that of
light in an optical telescope, as discussed in the answer to Question 6.
A three-dimensional corner reflector produces an image that is inverted.
To see this,
imagine a corner reflector at about waist level.
Light from your head approaches the
reflector moving downward.
After reflecting, the light from your head moves on a
parallel path but in the opposite direction; that is, it now moves upward.
from your feet moves upward before reflection, but downward after reflection.
the reflected light from your head moves upward, and the reflected light from your feet
moves downward, it follows that the image of your head is below the image of your feet –
your image is inverted.
Light bends toward the normal when it enters a medium in which its speed of
propagation is less than it was in the first medium – as when light passes from air to water.
On the other hand, light bends away from the normal if it enters a medium in which its
speed is increased – as when light passes from water to air.
When light goes from air to glass it slows down; when it goes from glass to air it speeds
In general, the speed of light is determined solely by the medium in which it
propagates, irrespective of its past history.