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 Moon’s 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 explanations.

Optical physics
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 mirages.

   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 simplicity’s 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 Newton’s 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. “twinkling” stars..

   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.

Atmospheric Optics

   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 of color.

   Imagine entire walls of raindrops all reflecting different wavelengths of light at different points on the Earth’s 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 Space.”

   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 sun’s 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 sun’s 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 sun’s 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 sun’s 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 sun’s rays hit the Earth almost parallel to one another, then what is going on? We are being fooled by the optical illusion of da Vinci’s 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 Earth’s 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 here to read Part II

Cynta deNarvaez