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Page 36 of 160 (1595 Questions and Answers)

351. What makes the clouds white - or having colors at sunset and why is the sky gray on a cloudy day?
The water droplets in clouds are quite large; large enough to be good antennas for all colors of light. As light passes by those droplets, some of it scatters (is absorbed by the antenna/water droplets and is reemitted by the antenna/water droplets). Since there is no color preference in this scattering from large droplets, the scattered light has the same color as the light that illuminated the cloud. In the daytime, the sunlight is white so the clouds appear white. But at sunrise or sunset, the sun's light is mostly red (the blue light has been scattered away by the atmosphere before it reached the clouds) so the clouds appear red, too. If the clouds are very thick, they may absorb enough light (or scatter enough upward into space) to appear gray rather than white. Another way to see why the clouds are white is to realized that light reflects from every surface of the water droplets. As the light works its way through the random maze of droplets, it reflects here and there and eventually finds itself traveling in millions of random directions. When you look at a cloud, you see light coming toward you from countless droplets, traveling in countless different directions. You interpret this type of light, having the sun's spectrum of wavelengths but coming uniformly from a broad swath of space, as being white. These two views of how light travels in a cloud (absorption and reemission from droplets or reflections from droplet surfaces) turn out to be exactly equivalent to one another. They are not different physical phenomena, but rather two different ways to describe the same physical phenomena.

352. When I look up at the sky on a clear day, there is the sun, then a surrounding circle of white-blue light covering maybe half the sky, encircled by deep blue down to the horizon, followed by a white layer at the horizon itself. Please explain these zones.
The ring that you see surrounding the sun is probably the 22° halo caused by refraction from ice crystals in the upper atmosphere. These tiny ice crystals are hexagonal prisms and they deflect the light that passes through them to form a ring of light around the sun. Because the particles are large enough to bend all the colors of light equally, the ring appears white—or blue-white when superimposed on the blue sky. The deep blue of the surrounding sky is caused by Rayleigh scattering of the sunlight passing through it. In this process, small groups of air molecules and tiny dust particles deflect sunlight toward your eye. Since they deflect short wavelength light (blue light) more effectively than long wavelength light (red light), they give the sky a bluish glow. Finally, the white appearance of the horizon is probably light scattered toward your eyes by surface haze. Relatively large particles in the air scatter sunlight in all directions so that you see a white glow from the air near the ground.

A wonderful reference for some of these ideas is "Rainbows, Halos, and Glories" by Robert Greenler.

353. Why are tanning beds not good for you; also there are some new ones recently that claim that they are safer than others (have no B rays)? Are they about the same as the sun itself or how much worse for you?
Tanning beds emit ultraviolet light in order to trigger your skin's tanning response. This ultraviolet light can and does cause chemical damage to your skin. Like all light, ultraviolet light is absorbed and emitted as particles. The energy in each light particle depends on its wavelength and, since ultraviolet light has short wavelengths, ultraviolet light particles carry lots of energy. They carry enough energy to rearrange the molecules that absorb them. If those molecules are part of the genetic information of a cell, the cell may die or, worse yet, may become cancerous. The shorter the wavelength of the ultraviolet light, the more energetic its particles and the more damage it can do. Tanning beds walk a narrow line between inducing tanning and causing significant damage. Leather skin is one end result of too much chemical damage. Tanning beds that emit relatively long wavelength ultraviolet are probably less harmful than those that emit shorter wavelength ultraviolet (these wavelength ranges are sometimes designated by letters A, B, and C...I think that A is the longest wavelength and least harmful). Still, you skin's tanning response is a defense against chemical damage and is probably not worth trying to trigger with light. Recent research seems to have found chemicals that trigger tanning. These chemicals mimic light-damaged molecules in your skin. Your skin senses these molecules and responds by tanning. If these chemicals work, you'll soon be able to develop a true tan without exposure to light.

354. Why are there sunspots?
The sun is a ball of incandescent gas. That gas moves about, flowing up and down as well as across the sun's surface. This movement keeps the sun's temperature roughly uniform but there are occasionally imperfections; regions of the sun's surface that get out of balance with the rest of the sun. When you cook a thick soup on the stove, there will also be regions of the surface that are cooler than others.

355. Why can water appear brown, blue (as in the ocean), and clear (as in a glass of water)?
Brown water contains colored contaminants that provide the color. Brown is the typical end result for a random mixture of pigments. The blue ocean is caused mostly by the sky. Since the ocean reflects some of the light from the sky, it appears blue. Pure water is almost completely colorless. Thus a glass of water has no color (unless you illuminate it with colored light). But if you look at a white light through many meters of water, that light will become slightly colored. Water absorbs a very small amount of visible light and you will see only what is not absorbed. I'm not sure what color pure water has. It may appear slightly green.

356. Why do dark clothes absorb heat more than light clothes?
Dark fabrics or surfaces are very good at absorbing and emitting light. That is why they are dark. They must contain electric charges that move fairly easily (making them good antennas) and these charges must be good at exchanging energy with the surrounding material as heat. When light strikes these charges, the charges begin to move and absorb the light's energy. This energy flows into the material as heat. Since the light is absorbed, the material appears dark (no light is reflected back toward you). But the material will also emit light very effectively when hot. If you heat a black object up, heat will flow into the charges, which will begin to move and will emit light. Thus black objects are good at both absorbing and emitting light.

357. Why do different sunglasses appear darker than others?
Polarizing sunglasses block half the light (stopping horizontally polarized light and passing only vertically polarized light). But sunglasses of all types contain chemicals that absorb light of both polarizations. The darkness of the sunglasses depends on which chemicals are used and how much of those chemicals they contain. Some sunglasses are also coated with thin metallic layers that reflect a fraction of the light that strikes them. These semi-transparent mirrors can change the transmission of the sunglasses dramatically so that those sunglasses may transmit 50% of the light or 0.01% of the light. The manufacturer can choose.

358. Why do fine mists of water create rainbows?
Fine mists of water are basically spherical water droplets in air and these can produce rainbows in exactly the same manner as raindrops do in natural rainbows.

359. Why do sunspots affect radio and TV reception?
Although I do not really know very much about the connection between sunspots and radio reception, I believe that the problem lies in with the solar wind. The solar wind is a steady stream of electrically charged particles that is responsible for the aurora, among other things. Since charged particles that interact with the earth's magnetic field accelerate, they emit radio waves. These waves should cause reception problems on earth. If anyone reading this knows otherwise or has more information, please let me know.

360. Why do you sometimes see a circular rainbow surrounding a light?
It is most often caused by the bending of light by mist around the light or by flaws in the optical components through which you are viewing the light. Whenever light passes through a clear material, its path bends. In most cases, you only notice that the light is distorted by its passage through the material. But different colors (wavelengths) of light bend by slightly different amounts so that the colors of light sometimes appear to come from slightly different directions. That's the origin of the rainbow you see.
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