. I once saw a green sunrise. Can you explain this?
Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed) in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html
. According to the author of that site, Andrew Young, given a low enough horizon, which is the primary consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly see green flashes at most sunsets.
. How can I clean a dirty CD which has a very difficult to remove stain? Which materials are best for cleaning? — AM, Mexico
Most CD's are made from polycarbonate plastic (though other plastics with the same index of refraction are occasionally used). Polycarbonate is a pretty tough material, so it should survive most common stain or gum removing solvents. Try your favorite solvent on an unimportant CD first; such as one of the free discs that come occasionally in the mail. However, if the stain molecules have diffused into the plastic and have become trapped within the tangle of plastic molecules, you're probably out of luck. Removing such a stain will require wearing away some of the plastic. Since the disc's surface finish must remain smooth and the thickness of the disc shouldn't change much, serious resurfacing is likely to make the disc unplayable. Also, stay away from the printed side of the disc—it has only a thin layer of varnish protecting the delicate aluminum layer from injury. Solvents can wreck this side of the disc. Finally, if the stain is a white mark (or a scratch), you may be able to render the disc clear again by filling the tiny air gaps that make it white with another plastic. I'll bet that a clear furniture polish or liquid wax will soak into the white spot, replace the air, and render the disc clear and playable.
. How does a fan compare to a propeller? Why does a fan blow air while a propeller has "lift"? — DB, Austin, TX
A fan and a propeller are actually the same thing. Both are rotating wings that push the air in one direction and experience a reaction force in the opposite direction as a result. Each experiences a "lift" force, typically called "thrust," in the direction opposite the airflow. If you put a strong fan on a low-friction cart or a good skateboard, it will accelerate forward as it pushes the air backward. Similarly, if you prevent a propeller plane from moving, its spinning blades will act as powerful fans.
. You stated (elsewhere) that thermodynamics overwhelms just about everything sooner or later. Could you explain why? — MT, San Antonio, TX
One of the principal observations of thermodynamics (and statistical mechanics, a related field) is that vast, complicated systems naturally evolve from relatively unlikely arrangements to relatively likely arrangements. This trend is driven by the laws of probability and the fact that improbable things don't happen often. Here's an example: consider your sock drawer, which contains 100 each of red and blue socks (it's a large drawer and you really like socks). Suppose you arrange the drawer so that all the red socks are on one side and all the blue socks are on the other. This arrangement is highly improbable—it didn't happen by chance; you caused it to be ordered. If you now turn out the light and randomly exchange socks within the drawer, you're awfully likely to destroy this orderly situation. When you turn the light back on, you will almost certainly have a mixture of red and blue socks on each side of the drawer. You could turn the light back out and try to use chance to return the socks to their original state, but your chances of succeeding are very small. Even though the system you are playing with has only 200 objects in it, the laws of probability are already making it nearly impossible to order it by chance alone. By the time you deal with bulk matter, which contains vast numbers of individual atoms or electrons or bits of energy, chance and the laws of probability dominate everything. Even when you try to impose order on a system, the laws of probability limit your success: there are no perfect crystals, perfectly clean rooms, flawless structures. These objects aren't forbidden by the laws of motion, they are simply too unlikely to ever occur.
. How does a rice cooker know when to turn off? — JS, Tokyo, Japan
The rice cooker turns off when there is no longer enough liquid water on its heating element to keep that element's temperature at the boiling temperature of water (212° F or 100° C). As long as the element is covered with liquid water, it is hard for that element's temperature to rise above water's boiling temperature. That's because as the water boils, all of the thermal energy produced in the heating element is converted very efficiently into chemical potential energy in the resulting steam. In short, boiling water remains at 212° F even as you add lots of thermal energy into it.
But as soon as the liquid water is gone (and, fortuitously, the rice is fully cooked), there is nothing left to keep the heating element's temperature from rising. As more electric energy enters the element and becomes thermal energy, the element gets hotter and hotter. A thermostat, probably a bimetallic strip like that used in most toasters, senses the sudden temperature rise. It releases a switch that turns off the electric power to the rice cooker.
. How does a VCR Plus system work? Are codes built in for every possibility of channel and time or does it calculate somehow? I know that if you enter a random number (including single digits) that some program is scheduled. — LK, Huntington, West Virginia
The VCR Plus codes contain just enough information to tell the VCR what time and day a program starts, what channel that program is on, and how long it will last. What is remarkable about these codes is not that they exist, but that many of them are so short. A long number that contained the complete date, the entire channel number, and the length of the program in minutes would obvious fulfill the requirements, but the actually numbers are never that long. While I don't know the precise encoding scheme, the date is clearly compressed—a daily or weekly program is represented by a very small code—and so is the record time for programs with a common duration. The VCR Plus codes get significantly longer when they must represent one-time only shows and shows with complicated durations. Even then, the date is truncated so that there are no current codes to represent a show five years in the future.
. I work finding sites for cellular & PCS wireless telephone antennae. I would like to know how radio waves work and how they are able to carry voice and data information. What are these waves and do they exist naturally or do we set them up using electric charges? — PAB, Madison, WI
Radio waves are a class of electromagnetic waves, specifically the lowest frequency, longest wavelength electromagnetic waves. Actually, the electromagnetic waves used in cellular & PCS transmissions are technically known as microwaves because they have wavelengths of less than 1 meter, but there are no important differences between radio waves and microwaves.
Like all electromagnetic waves, radio waves and microwaves consist of coupled electric and magnetic fields that sustain one another in stable structures that move rapidly through empty space. Because an electromagnetic wave's electric field changes with time, it is able to create the wave's magnetic field and, because its magnetic field changes with time, that magnetic field is able to create the wave's electric field. Since they consist only of electric and magnetic fields, these waves cannot stay still—they must move (although you can trap them between mirrors so that they appear to stand in one place as they bounce back and forth). While they contain no true mass, they do contain energy and an electromagnetic wave carries energy from one place to another.
Electromagnetic waves are created whenever electrically charged particles change speed or direction; whenever they accelerate. Since there are accelerating electric charges everywhere—thermal energy keeps them moving about—there are also electromagnetic waves everywhere. But the radio waves used in communications systems are generated deliberately by moving electric charges back and forth. When charges are sent up and down a radio antenna, these charges are accelerating and they form complicated electric and magnetic fields that include electromagnetic waves. Once launched, those electromagnetic waves propagate through space at approximately the speed of light.
To send information with radio waves, a transmitter makes modifications in one or more the wave's characteristics. In an amplitude modulation scheme (AM), the transmitter changes the strength or "amplitude" of the wave to convey information—like sending radio smoke signals. In the frequency modulation scheme (FM), the transmitter changes the frequency of the wave to convey information—like whistling a tune with a complicated melody.
. I know that the medium of electromagnetic waves is a photon. What is a photon? What is it made of? — ON, Istanbul, Turkey
First, an electromagnetic wave consists of an electric and a magnetic field. These two fields create one another as they change with time and they travel together through empty space. An electromagnetic wave of this sort carries energy with it because electric and magnetic fields both contain energy. That much was well understood by the end of the 19th century, but something new was discovered at the beginning of the 20th century: an electromagnetic wave cannot carry an arbitrary amount of energy. Instead, it can carry one or more units of energy, units that are commonly called "quanta." An electromagnetic wave that carries only one quanta of energy is called a "photon."
The amount of energy that a photon carries depends on the frequency of that photon—the higher the frequency, the more energy. Photons of visible light carry enough energy to induce various changes in atoms and molecules, which is why they provide our eyes with such useful information about the objects around us—we see how this visible light is interacting with the world around us.
. In science, we learned that a color's energy depends on its wavelength—that violet light with its short wavelength has more energy than red light with its long wavelength. But in art, we learned that red, orange, and yellow are warm and blue and violet are cool. Is that because of how the people feel about the colors, like fire is red and water is blue? — ON, Istanbul, Turkey
Both of your observations are correct: short wavelength light, such as violet, carries more energy per particle (per "photon") than long wavelength light, such as red, and red light does appear "warmer" than blue light. But the latter observation is one of feelings and psychology, rather than of physics. It is ironic that colors we associate with cold and low thermal energies are actually associated with higher energy light particles than are colors we associate with heat and high thermal energies.
. Is there a formula or equation for figuring out the pressure of air at a certain altitude? — DLH, Conifer CO
Unfortunately, the answer is no. The atmosphere is too complicated to be described by a simple formula or equation, although you can always fit a formulaic curve to measured pressure values if you make that formula flexible enough. The complications arise largely because of thermodynamic issues: air expands as it moves upward in the atmosphere and this expansion causes the air to cool. As a result of this cooling, the air in the atmosphere doesn't have a uniform temperature and, without a uniform temperature, the air's pressure is difficult to predict. Radiative heating of the greenhouse gases and phase changes in the air moisture content further complicate the atmosphere's temperature profile and consequently its pressure profile. If you want to know the air pressure at specific altitude, you do best to look it up in a table.