Geoscience Reference
In-Depth Information
Figure 7.18.
Rainbow over an Alaskan lake in
September 1992. Commander John Bortniak, NOAA
Corps, available from the National Oceanic and
Atmospheric Administration Central Library;
www.photolib.noaa.gov/.
Figure 7.17.
Forward and side scattering of sunlight
by a cloud. The thickness of the cloud prevents most
sunlight from transmitting through it. Photo by Mark
Z. Jacobson.
Thus, particles scatter light primarily in the forward
direction. They also scatter some light to the side and
backward.
Backscattered
light results primarily from
asingle internal reflection (ray E). The light rays seen
in Figure 7.17 are the result of light scattering off cloud
drops in the forward and sideward directions.
arainbow appears when individual waves from many
raindrops hit the viewer's eye. As seen in Figure 7.19,
red appears on the top of a primary rainbow. A sec-
ondary rainbow occurs if a second reflection occurs
inside each raindrop.
Because winds at midlatitudes originate from the
west or southwest (Figure 6.5) and a rainbow appears
only when the sun is at a viewer's back, sailors at mid-
latitudes knew that if they saw a rainbow in the morning,
the rainbow was to the west and the winds were driving
the storm creating the rainbow toward them. If they saw
arainbow in the evening, the rainbow was to the east
and the winds were driving the storm creating the rain-
bow away from them. These factors led to the following
rhyme:
7.1.4.5. Rainbows
Arainbow results from two light-scattering processes,
dispersive refraction and reflection, and can be seen
only if the sun is at the viewer's back and raindrops are
falling in front of the viewer. The seven most prominent
colors in a rainbow are red, orange, yellow, green, blue,
indigo, and violet. In a
primary rainbow
,red appears
on the top and violet appears on the bottom. Figure 7.18
shows an example of a primary rainbow. In a
secondary
rainbow
,sometimes seen faintly above a primary rain-
bow, violet appears on top and red appears on the bot-
tom. For convenience, the discussion of rainbows below
considers only red, green, and blue wavelengths.
Figure 7.19 shows how light interacts with raindrops
to form a primary rainbow. As a beam of visible light
enters a raindrop, all wavelengths bend toward the sur-
face normal due to refraction. Blue light bends the most
as a result of dispersive refraction. When light hits the
back of the drop, much of it reflects internally. When
the reflected light hits the front edge of the drop, it
leaves the drop and refracts away from the surface nor-
mal. The angles of the blue and red wavelengths that
reach a viewer's eye are 40 and 42 degrees, respectively,
in relation to the incident beam. Only one wavelength
from each raindrop impinges upon a viewer's eye. Thus,
Rainbow in the morning, sailors take warning,
Rainbow at night, sailor's delight.
Visible
radiation
42°
40°
Blue
Red
Blue
Red
Figure 7.19.
Geometry of a primary rainbow.
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