Page 206 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch07_177-202.indd Page 183 9/1/10 9:43 PM user-f465
tiL12214_ch07_177-202.indd Page 183 9/1/10 9:43 PM user-f465 /Users/user-f465/Desktop

same rate that they were spreading before reflection. Your eyes CONCEPTS Applied
and brain do not know that the rays have been refl ected, and
the diverging rays appear to come from behind the mirror, as Good Reflections

the dashed lines show. The image, therefore, appears to be the
Do you need a full-length mirror to see your hair and your feet
same distance behind the mirror as the block is from the front
at the same time? Does how much you see in a mirror depend

of the mirror. Thus, a mirror image is formed where the rays of
on how far you stand from the mirror? If you wink your right
light appear to originate. This is called a virtual image. A virtual eye, does the right or left eye of your image wink back?

image is the result of your eyes’ and brain’s interpretations of light You can check your answers to the above questions
rays, not actual light rays originating from an image. Light rays with a large mirror on a wall, masking tape, a meter stick,
that do originate from the other kind of image are called a real and someone to help.
image. A real image is like the one displayed on a movie screen, Stand facing the mirror and tell your helper exactly
with light originating from the image. A virtual image cannot where to place a piece of masking tape on the mirror to
be displayed on a screen, since it results from an interpretation. mark the top of your head. Place a second piece of tape on
Curved mirrors are either concave, with the center part the mirror to mark the top of your belt or some other part
of your clothing.
curved inward, or convex, with the center part bulging out-
Now we need some measurements on you and on the
ward. A concave mirror can be used to form an enlarged virtual
mirror. First, measure and record the actual distance from
image, such as a shaving or makeup mirror, or it can be used to
the top of your head to the top of your belt or the other
form a real image, as in a refl ecting telescope. Convex mirrors, clothing marked. Second, measure and record the distance
such as the mirrors on the sides of trucks and vans, are oft en between the two pieces of masking tape on the mirror.

used to increase the field of vision. Convex mirrors are also used Now step back, perhaps doubling your distance
in a driveway to show a wide area (Figure 7.13). from the mirror, and repeat the procedure. What can you
conclude about your distance from the mirror and the
image you see? Is there a difference in how much you can
see? How tall a mirror do you need to see yourself from
head to toe? Finally, is there some way you can cause your
image to wink the same eye as you?

REFRACTION
You may have observed that an object that is partly in the air
and partly in water appears to be broken, or bent, where the
air and water meet. When a light ray moves from one trans-
parent material to another, such as from water through air, the
ray undergoes a change in the direction of travel at the bound-

ary between the two materials. This change of direction of a

light ray at the boundary is called refraction. The amount of
change can be measured as an angle from the normal, just as
it was for the angle of refl ection. The incoming ray is called the

incident ray as before, and the new direction of travel is called
the refracted ray. The angles of both rays are measured from the

normal (Figure 7.14).
Refraction results from a change in speed when light passes

from one transparent material into another. The speed of light
8
in a vacuum is 3.00 × 10 m/s, but it is slower when moving
through a transparent material. In water, for example, the speed
8
of light is reduced to about 2.30 × 10 m/s. The speed of light

has a magnitude that is specific for various transparent materials.
When light moves from one transparent material to
another transparent material with a slower speed of light, the
ray is refracted toward the normal (Figure 7.15A). For example,
light travels through air faster than through water. Light travel-
ing from air into water is therefore refracted toward the normal
FIGURE 7.13 This convex mirror increases your field of vision, as it enters the water. On the other hand, if light has a faster
enabling you to see merging cars from the left. speed in the new material, it is refracted away from the normal.

7-7 CHAPTER 7 Light 183