Effects of Pollution on Visibility, Ultraviolet Radiation, and Colors in the Sky - Air Pollution and Global Warming: History, Science, and Solutions - page 163

Apparent
position

Particle

Diffracted rays

Actual
position

Source

A

Earth

Primary
wavefronts

Secondary
wavefronts

Figure 7.14. Refraction of starlight by the
atmosphere makes a star appear to be where it is not.

Figure 7.15. Diffraction of light around a spherical
particle. Any point along a wavefront may be taken as
the source of a new series of secondary waves. Rays
emitted from point A appear to cause waves from the
original source to bend around the particle.

45 ◦ , air to liquid water at an incident angle of 1 =

wavelengths of 0.4 and 0.7 mare bent by 13.11 and

12.90 degrees, respectively. Thus, refraction bends short
(blue) wavelengths of visible light more than it bends
long (red) wavelengths. Separation of white visible light
into individual colors by this selective refraction is
called dispersion (or dispersive refraction ). When Sir
Isaac Newton separated white light into multiple colors
by passing it through a glass prism (Section 7.1), he
discovered dispersive refraction.
As shown in Figures 7.6 and 7.8, refraction between
space and the atmosphere is responsible for twilight,
which is the sunlight seen after the sun sets and before
the sun rises. Such refraction also causes stars to appear
positioned where they are not, as shown in Figure 7.14.
Layers of air at different densities in the Earth's atmo-
sphere cause starlight to refract multiple times, and thus,
flicker , twinkle ,or scintillate .

wavefront. The diffracted rays in Figure 7.15 appear to
cause light to bend around the obstacle.

7.1.4.4. Summary of Particle Scattering
Particle scattering is the combination of the effects
of reflection, refraction, and diffraction. When a wave
approaches a spherical particle, such as a cloud drop,
it can reflect off the particle, diffract around the edge
of the particle, or refract into the particle. Once in the
particle, the wave can be absorbed, transmitted through
the particle and refracted out, or reflected internally
one or more times and then refracted out. Figure 7.16
illustrates these processes, except for absorption, which
is not a scattering process. The processes that affect
particle scattering the most are diffraction and dou-
ble refraction, identified by rays C and B, respectively.

7.1.4.3. Diffraction
Diffraction is a process by which the direction of prop-
agation of a wave changes when the wave encounters
an obstruction. In terms of visible wavelengths, it is the
bending of light as it passes by the edge of an obstruc-
tion. In the air, waves diffract as they pass by the surface
of an aerosol particle, cloud drop, or raindrop.
Diffraction can be explained in terms of Huygens's
principle ,which states that each point of an advancing
wavefront may be considered the source of a new series
of secondary waves. If a stone is dropped in a tank
of water, waves move out horizontally in all directions,
and wavefronts are seen as concentric circles around the
stone. If a point source emits waves in three dimensions,
wavefronts are concentric spherical surfaces. When a
wavefront encounters the edge of an obstacle, waves
appear to bend (diffract) around the obstacle because
aseries of secondary concentric waves is emitted at
the edge of the obstacle along the advancing wavefront
(e.g., in Figure 7.15) and at each other point along the

A Sidescattering

B

Forward
scattering

C

E
Backscattering

D Sidescattering

Figure 7.16. Radiative scattering by a sphere. Ray A
is reflected; B is refracted twice; C is diffracted;
Disrefracted, internally reflected twice, and then
refracted; and E is refracted, reflected once, and then
refracted. Rays A-D scatter in the forward or
sideward direction, whereas E scatters in the
backward direction.