Chapter 16 Notes
Optical Phenomena
of the
Atmosphere
A Rainbow
The Nature of Light
The light that forms the array of colors that constitute the
rainbow, or the deep blue color of the sky, originates as visible
light from the Sun. The interaction of sunlight with the
atmosphere creates the numerous optical phenomena that take
place in the sky.
Light from the Sun interacts with the gases of the atmosphere,
as well as with ice crystals and water droplets, to create the
optical phenomena through the properties of reflection,
refraction, diffraction and interference.
Reflection of Light by a
Smooth Surface
Reflection of Light
by a Rough Surface
Reflection
Light traveling through space travels in a straight line and at a constant
speed. When the light enters a transparent medium, like glass, most of
the light will travel through the glass at slower speed. Some of the light,
however, will bounce off of the surface of the medium; this bouncing back
of the light is called reflection.
The law of reflection – the angle at which the light strikes a surface or
boundary and the angle at which the light is reflected are equal. If the
surface or boundary is not smooth, light will strike and reflect at different
angles.
Internal reflection – occurs when light that is traveling through a
transparent material reaches the opposite surface and is reflected back in
the transparent material. It is an important factor in the formation of
optical phenomena, such as rainbows.
Refraction of Light
Refraction
Refraction is the bending of light as it passes obliquely from one transparent medium to
another.
Refraction is caused because the velocity of light varies depending on the material that
transmits it. In a vacuum, radiation (including light) travel at 3.0x108 meters per second,
however other transparent media the speed of light will vary depending on the density of
the medium through which it is traveling. The speed of light in a medium is always
less than the speed of light in a vacuum.
The bending of light by refraction is responsible for a number of common optical
illusions. These illusions occur because our brain perceives bent light as if it has
traveled to our eyes along a straight path.
Refraction Causes Sun to Appear Higher
Hot Air Near Surface Refracts Light to Invert Images
Mirages
Mirage – an optical effect of the atmosphere caused by refraction in which the
image of an object appears displaced from its true position, Types of mirages
include inferior images, superior images, and towering.
Mirages occur when there is a significant difference in the temperature air
near the ground to temperature of the air aloft.
Refraction over Air Cooled by Water Makes Ship Appear to Float in Air
Rainbow
Rainbows
An observer on the ground sees the rainbow as an arch-shaped
array of colors that trail across a large segment of the sky. Two
conditions are required for a person to see a rainbow:
1) The Sun must be opposite of a rain shower
2) The observer must be between the sun and the rain shower
Rainbows are caused be the refraction of sunlight as it passes
through water droplets suspended in the sky. Each color of light
travels at a different velocity in water; consequently, each color
will be bent at a slightly different angle. Red light travels fastest
through the water and is bent the least, while violet light travels
slowest and is bent the most. The separation of sunlight into the
colors of the rainbow is called dispersion.
Halo around Sun Caused by Cirrostratus Clouds
Halos, Sun Dogs, Solar Pillars
Halos appear as a narrow whitish ring having a large diameter centered on
the Sun. Halos occur on days when the sky is covered with a thin layer of
cirrus clouds, and is more often viewed in the morning or late afternoon when
the Sun is near the horizon.
Sun dogs (or parhelia) are associated with halos and are seen as two bright
regions on either side of the Sun
Sun pillars are vertical shafts of light that are most often viewed near sunset
or sunrise, and appear to extend upward from the Sun.
Sun Dogs (Parhelia)
Halos, sun dogs, and sun pillars are
caused by the dispersion of sunlight by
the shape and motion of ice crystals in
the atmosphere.
Ice crystals occur in four basic shapes:
(a)plates, (b) columns, (c) capped
columns, and (d) bullets.
Sun Pillar
The Corona
Corona
More commonly associated with the Moon
than the Sun, it is a bright whitish disk
centered on the Moon or Sun. A corona is
produces when water droplets in a thin layer
of water-laden clouds, usually altostratus,
scatter light from the illuminating body.
The colors of the corona are the result of
diffraction, the slight bending of light as it
passes near the edges of cloud droplets, and
the interference of the various components
of white light.
A Glory
The Glory
Most commonly seen by pilots, a glory consists of one or
more colored rings that surround the aircraft’s shadow
projected on the clouds below. The glory forms much like
the rainbow, but the water droplets are much smaller and
more uniform in size.