March 22, 1995
One Minute Papers - Questions and Answers
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.
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.
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.
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.
Why is it any worse to observe a solar eclipse rather than a normal glimpse at the sun?
The problem with looking at the sun during a solar eclipse is not that it is somehow brighter than normal but rather that (1) you tend to stare at it and (2) the size of its bright region is reduced so that it does "hurt" as much to stare at it. It's hard to stare at the full sun because it feels uncomfortable but looking at a tiny part of the sun may not feel bad enough to make you avert your eyes. Nonetheless, that tiny part of the sun can cook your retina and cause permanent damage.
Why do rainbows occur when water is dispersed on a sunny day?
That is one of the topics for the next lecture.
Why is a blue flame hotter than a red flame?
The colors of flames can be deceiving because they involve emissions from particular atoms (which impart their own characteristic colors to the light they emit). However, a blue-hot object such as a star is hotter than a red-hot object such as a glowing coal in the fireplace.
How does light create heat?
Actually, some light is heat. Heat is the energy that flows from one object to another because of a difference in their temperatures. The sun is hotter than you are so that it sends heat toward you. Sunlight is heat; it is the sun's heat begin sent toward you as electromagnetic radiation. When it strikes the surface of your skin, this radiation is absorbed and becomes the more familiar form of heat: kinetic and potential energy in the atoms and molecules. From the surface of your skin, this heat flows inward to warm the rest of your body. Any material that absorbs light usually converts it to heat. The charged particles in that material move under the influence of the light's electric field and these moving charged particles transfer their energy here and there as heat.
Does air pollution contribute to the blueness of the sky (make it bluer)? Has the sky become more blue with the advent of technology (factories, machinery, etc.)?
Yes. Pollution does tend to make the sky bluer and the sunsets redder. However, pollution also imparts colors directly by absorbing certain wavelengths of light. The orange haze that hovers over cities is often caused by nitrogen oxides, which are simply orange in color and act like pigments to make everything appear orangish. However smoke and dust certainly change the look of the sky by increasing scattering. Natural disasters are even more effective: volcanic eruptions create the most beautiful sunsets of all by tossing vast amounts of dust into the air.
What is black light and how does it work?
Black light is ultraviolet light. You cannot see it so a room illuminated only by ultraviolet light appears dark or "black". However any fluorescent materials in the room (e.g. brighteners in your clothes) will absorb the ultraviolet light and reemit it as visible light. That is why things with fluorescent pigments on them glow when illuminate by black light.
Why do camera flashes make eyes red and why do two flashes correct this problem?
The retinas of your eyes appear reddish when you look at them with white light. The red eye problem occurs because light from the flash passes through the lens of your eye, strikes the retina (which allows you to see the flash), and reflects back toward the camera. This reflection is mostly red light and it is directed very strongly back toward the camera. The camera captures this red reflection very effectively and so eyes appear red. The double flash is meant to get the irises of your eyes to contract (as they do whenever your eyes are exposed to bright light or you are startled or excited). The first flash causes your irises to contract so that less light from the second flash can pass into and out of your eyes. Unfortunately, this trick doesn't work all that well.
Why are there sun spots?
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.
Why do sun spots affect radio and TV reception?
Although I do not really know very much about the connection between sun spots 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.