Yeah, But How?
A brief foray into atmospheric optics
I have been fascinated with the obvious of late. In
the past I have seen things I could name, and because I could name them or had some vague
understanding of their processes, I left my curiosity at the door. Well, not any more!
After experiencing the meager monsoon display this summer, I began missing the atmospheric
drama that normally accompanies August - especially rainbows. As I started looking into
rainbows, other skyward optical phenomena that had been nagging, unanswered questions in
the past became interesting once again. Why is the sky blue? Why is the Moons sky
always dark? How do stars twinkle? Why does the sun flatten out when it sets? Why is there
a puddle of water seen with mirages? Is there really such a thing as The Green
Flash? And what in tarnation is lightning?
To understand these and other light-oriented concepts, several basic physical
laws and processes must be defined. First the definitions, then the descriptions and
Basic properties of light within our atmosphere
Light and waves: Isaac Newton discovered that in a vacuum
light waves travel with maximum velocity and in a straight line. Light is emitted
(radiated) from the sun and travels in a straight line until it hits an obstruction like
our atmosphere. It is the reflection, refraction and diffraction of these waves that cause
us to see atmospheric optical displays like rainbows, green flashes and earthbound
Light waves have different lengths or frequencies. There is still
a question as to whether these waves are really only waves of
energy, or are actually particles as Einstein contends; for they
seem to have the properties of both. For simplicitys sake, we will call them waves
here. Within the visible range, light beams appear white, yet when they are dispersed we
see a display of color.
Newton took a beam of light and shined it into a prism. The beam travelled in
a straight line until it hit the glass of the prism, a new medium. Once it hit
this new medium, the light beam deflected with colors splayed at predictable angles. It
then continued on in a new direction until it hit the other side of the prism. As the beam
reacted to this different medium, glass to air, it dispersed the colors within the light
beam even more into a spectrum. This color, or electromagnetic spectrum,
displays the full range of visible wavelengths of light in order; bordered by ultraviolet
below and infra-red on top.
Reflection: We witness reflection in mirrors, on polished
surfaces and on water. Images of objects are bounced back from a medium because none of
the light waves were absorbed by that medium.
Refraction: This is the process of Newtons beam of
light through the prism. The speed of the beam, or wave, depends upon the properties of
the medium. The speed of the beam through air is faster than through glass (a denser
medium) so the light bends. because the refractive indices of
these media are different. The shorter waves, violet and blue, are refracted more than the
long red waves, and there is a separation of colors. Refraction is the unidirectional
bending and slowing of the light energy after it has come in contact with an obstruction
(like a new medium or level of pressure).
Diffraction: This is when a wave gets squeezed together, as
though through a small lens (like a water drop) and, when exiting, is diffused or spread
out. Rather than the wave remaining in its wavelike pattern, it gets broken up - like the
white light beam entering the prism.
Radiation: This is the spread of light from a center; the
emission of waves from a central point. In this instance, the distance from the Sun to the
Earth is such that the waves seem almost parallel as they hit the planet.
Inhomogeneity of atmosphere: Our atmosphere is made up of
air and air is made up of particles. The density of air is variable. The density is
affected by several factors: the barometric pressure; the amount of moisture in the air
(more moisture, more density), the altitude; air has weight and so is affected by the
gravitational pull of the Earth (every climber knows there is more air near the surface of
the Earth than at higher altitudes), and lastly, air is affected by temperature and
pressure (hot air expands, cold air compresses, hot air can absorb more water than cold
air, etc.). To make matters more interesting, air particles are continually colliding
against one another. Besides there being large levels and pockets of different pressures,
densities and moistures, there is a constant movement of pinballing particles causing
momentary blobs of air. So, unlike water, air is inhomogeneous; it fills its
space inconsistently; thickly, sparsely and/or turbulently. This inhomogeneity
affects the way light waves enter our system and, just like the prism, effects our optical
perception of light waves and the objects they attempt to represent, i.e.
Color and the way we see: Our eyes have been outfitted with
rods and cones with which we discern shape and color. Cones specify bright light and so
are color receptors while rods react to dim light and are shape or perspective detectors.
The colors we see are the absorption or reflection of specific wavelengths of light as
they strike an object. When the entire light wave is absorbed, we see black; the absence
of color. When the light wave is reflected we see white, light diffusing and all colors
overlapping. In many cases, different frequencies of light waves are reflected or absorbed
as a result of the chemical makeup of an object. For example, most plants appear green
because the pigment (chemical compounds in the skin of a plant) absorbs all the colors of
the light spectrum but the green frequency. So if you try to grow a plant using green
light, it will either change color, or if it grows at all, will do so feebly. The chemical
makeup of the minerals within the Hermit Shale absorbs all the wavelengths of light but
red. This red wavelength has been bounced back, reflected from the rock, and stimulates
the cone receptors of our retinas. Not all the colors we perceive are made by this
absorption or emission process. The color in the wings of blue birds has an entirely
different cause not to be propounded here. The sky also has its own reasons for being
blue, but this will be discussed later.
Rainbows: Rainbows are the large scale representation of
the refraction and reflection of light by raindrops. Descartes first figured this process
out in 1637 using glass spheres. As the beam of light first enters the raindrop (diameter:
200 micrometers), it refracts (is bent) and as the light diffuses, the colors separate and
head toward the opposite wall of the drop. Here, part of the light escapes out the back,
while the rest is reflected to the lower portion of the raindrop. Here it refracts even
more as the dispersed light finally exits and becomes a point within a great sky spectrum
Imagine entire walls of raindrops all reflecting different wavelengths of
light at different points on the Earths surface and one series hits you. A spectrum
is represented when higher droplets reflect red, orange below that, yellow below them and
so on. Violet and blue, of course are refracted most and so they are reflected from the
bottom portion of the rainbow. This single reflection event is called a primary
rainbow. These have the brightest images. For primary rainbows there is a consistent
angle of 42 degrees from Sun to top of rainbow (red color) to observer. And because the
angular diameter of a rainbow remains constant to the observer, we can never fit the
entire thing into a 35mm camera; as we step back to fit the full rainbow into the frame,
the color display moves with us.
Secondary rainbows occur when sunlight hits the rain at a higher angle and,
because of the angle of incidence, the light beam reflects twice while inside the droplet.
Secondary rainbows are always above primary ones. Their color spectrum is
reversed (violet and blue on top) because of the extra reflection. They have an angle of
51 degrees from Sun to rainbow to observer, and are fainter in appearance because more
light has had a chance to exit due to the second reflection. Supernumerary bows are the
faint arcs present within primary bows. These are simply interference bows; small
concentrations of minor light energy.
There is an inconsistency of brightness around rainbows. Rainbows are a
concentration of light through a raindrop, yet there is other visible light emerging
within the primary bow and above the secondary bow. This comes from the extraneous light
rays hitting all the droplets from every angle. There is a dark area between the rainbows
where the brightness has been reflected from. This is called the Alexandrian Dark
White rainbows: Occasionally these rainbows can be seen from
airplanes, but also from the ground in clouds and fog. The key is that the water droplets
must be quite small, 10 micrometers (or 10 millionths of a meter in diameter). This allows
for the diffraction of light. The light beam hits the small droplet, which acts like a
lens and squeezes the light in such a way that the bands of color within the beam spread
out and overlap, reflecting all the light energy and the color is received as white. To
witness one of these you must look 40 to 42 degrees from the top of the shadow of your
head on the ground. This is known as your antisolar point and is the domain of
all primary bows.
Red rainbows: These are placed high in the sky as the sun is
setting. As the sun descends, its rays travel through more of our atmosphere and as we
sequentially lose light waves (from violet to red) we witness a color display known as a
sunset. The intense reds of the sunset reflect off high clouds with a specific
diameter of raindrop - 10 micrometers. So when the sun is low on the horizon, an otherwise
white rainbow or cloud rainbow appears red because of the selective properties
of the atmosphere on the suns beams.
Lunar rainbows: According to Robert Greenler, Professor of
Physics at the University of Wisconsin-Milwaukee, there are lunar rainbows. However, to
produce enough light to have a rainbow, the Moon must be full. The light from the Moon is
a reflection of the suns rays and does not itself have the necessary intensity.
Needless to say, lunar rainbows are quite faint. Greenler says these bows have color but
appear white because of this lack of intensity. The process is the same as with solar
rainbows however; the same 42 degree angle, the same raindrops, only at night.
The deep blue sky: Violet/blue waves, being the smallest,
are more strongly refracted than red ones. Infrared waves are able to dodge atmospheric
particles while ultraviolet light gets pummeled and scattered by them. When
scattered, this blue light is reflected in all directions and we see
blue sky. For this same reason, Leonardo da Vinci, the father of perspective,
when asked how to put depth in a painting merely said, Add a little blue.
Seeing a long corridor of canyons corroborates this opinion.
It always puzzled me as a child why photographs on the Moon were always taken
during the evening. The Moon has no atmosphere (no air particles to scatter light), so its
sky always appears black.
Crepuscular and anticrepuscular rays: Occasionally, when the
sun is rising or setting, it is possible to see the suns rays looking like they have
been blasted through a colander. These are the same kinds of events pictured on the covers
of religious magazines. It looks as though the suns rays are emanating from a
central point directly within the cloud, not from a point 93 million miles away. If, as we
already know, the suns rays hit the Earth almost parallel to one another, then what
is going on? We are being fooled by the optical illusion of da Vincis point of
perspective; at distance all things converge to a central point. In this instance, the
central point of light is displaced due to the redirection of the solar beam. Crespuscular
rays occur when parallel solar beams are funneled down and redirected through clouds,
giving the impression that they are distinct beams emerging from a light source within the
cloud and that God is on the verge of speaking.
Anticrepuscular rays are only slightly different. Occasionally, when
crepuscular beams are visible, they can cover the entire sky. This does not mean that the
beams continue to fan out as they do from the (seemingly) original light source.
Anticrepuscular beams are the tail end of these beams, culminating at their own point of
perspective. As the light beams pass overhead, they converge on the horizon at exactly 180
degrees from the colander clouds.
Distortion of the rising and setting sun: Most of the time,
when the sun sets, we see a true image of that golden orb disappearing below the horizon.
Yet every now and then, when the conditions are right, we can witness a flattening of the
lower half of the sun. If we think of the Earths atmosphere as a series of flat
layers with increasing density due to gravitational pressure, the distortion of heavenly
bodies near the horizon becomes possible. Rays of light carrying the image of the sun are
bent at the points of entry to these layers due to changes in composition, pressure and
meteorological conditions. The air particles within the various layers are being
compressed by the weight of the air above as well as being affected by the gravitational
pull of the Earth. This increases the density within the lower layers which causes light
waves to bend toward the less dense air above. Simply speaking, when the sun is close to
setting, refraction will effect the top part of the sun differently from the bottom half.
The top half will radiate its image truly, while the bottom portion will send an apparent
image. Since the bottom portion of the sun is being seen through thicker, more dense
atmosphere, the bottom image is being bent intensely and gives the impression of being
squashed or flattened. The cool thing is that the bottom edge of the sun is
actually below the horizon and the bending of light lets us think its still above. It is a
little like an atmospheric pressure mirage.
end Part I
...click here to read Part II