Category: sunlight

Why is it that after swimming in a heavily chlorinated pool, you can see the spectrum around lights?

Your eye works very hard to keep all of the different wavelengths of light together so that they can form sharp images on your retina without any color errors. If you look at a white light bulb, all of the different colors from that bulb must arrive together on your retina or else you will see colors where they shouldn’t be. Keeping these colors together is no small task and is one of the biggest problems encountered by lens makers for cameras and telescopes. The chlorine in a pool evidently upsets your eye’s ability to control these color errors. However, I’m not sure what goes wrong or why chlorine causes this problem.

Does the rainbow go all the way to the ground?

Yes, it forms an arc that extends to the ground. However, any hills or valleys may obscure its visibility or its sunlight, so you often see it truncated or in shadow.

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.

Why isn’t the sky bright blue when the sun is red?

During the day, the sky is blue because the air and dust in the air scatter mainly blue light toward your eyes. They also scatter some red light, but the blue light dominates. But at sunset, things change. The setting sun approaches the earth’s atmosphere at a very shallow angle so that it must travel many kilometers through the air before reaching your eyes. During this long trip, most of the blue light is scattered away and the sun appears very red. If the path is long enough, the blue light is scattered away many kilometers to your west so that there isn’t much of it left. When this occurs, even the sky around you appears somewhat reddish because there just isn’t any more blue to scatter. The missing blue light is visible to people living 50 or 100 kilometers to the west as their blue sky.

How come I never find the pot of gold at the base of the rainbow?

The people who invented that tale were well aware of the impossibility of reaching the rainbow itself. Knowing that the rainbow moves with you, they were free to promise anything about what lies at the base of the rainbow.

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.

Why, if white doesn’t absorb heat, do I get very hot when I wear a white shirt?

A white shirt doesn’t absorb visible light (or at least very much visible light), but it may absorb lots of infrared light. Since much of the sun’s light and heat are in the form of invisible infrared light, that infrared absorption can be very important. There are many materials that appear white to your eye that do absorb strongly in the infrared and thus get very hot in sunlight.

How do oil spills/spots (i.e. in parking lots and streets) create rainbows?

A thin layer of oil on water creates interference effects, just like those seen in a thin soap film. Sunlight reflects from both the top and the bottom of the oil layer and these two reflections can interfere with one another. If the blue/green wavelengths of light interfere destructively on their way to your eye, you will see the oil layer as red. If the green/red wavelengths of light interfere destructively, you will see the oil layer as blue. How you see the oil layer depends on its thickness and the angles of the light.

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.

How do polarizing materials work?

The sheet polarizers that are used in sunglasses or in the demonstrations in class contain molecules that absorb electromagnetic waves of only one polarization. These molecules form long chains that interact with electromagnetic waves only when the electric fields push charge along the lengths of the molecules. In the polarizing sheets, the molecules are all oriented along the same direction so that they all absorb light of the same polarization. The other polarization of light passes through the sheets virtually unscathed. When unpolarized (randomly polarized) light enters one of these sheets, any waves that are polarized along the molecules are absorbed while any that are polarized across the molecules are permitted to pass. About half the light makes it through and that half is polarized across the molecules. If this remaining light is sent through a second polarizing sheet, turned 90° so that the molecules of the second sheet are aligned with the polarization of the light leaving the first sheet, then the remaining light will be absorbed in the second sheet and essentially no light will emerge from the pair of sheets. This arrangement, two polarizers turn 90° with respect to one another, is called “crossed polarizers”. It is a useful arrangement for observing materials that rotate polarization by distorting the electric and magnetic fields. If a distorting material is placed between the two crossed polarizers, light from the first polarizer may be altered by the material and thus be able to pass through the second polarizer.