Could you please give me a precise explanation of light scattering in relation t…

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Could you please give me a precise explanation of light scattering in relation to blue moons and red sunsets. Do dust particles, or whatever, facilitate the transmission of some wavelengths and not others? — DW

While the expression “blue moon” usually refers to the infrequent occurrence of second full moon in a calendar month, there have been rare occasions when the moon truly appeared blue. In those cases, an unusual fire or volcanic eruption filled the air with tiny clear particles that had just the right sizes to resonantly scatter away the red portion of the visible light spectrum so that only bluish light from the moon was able to pass directly to the viewer’s eyes. The moon thus appeared blue.

Red sunsets are much more common and they are caused by Rayleigh scattering—the non-resonant scattering of light by particles that are much smaller than the light’s wavelength. While Rayleigh scattering is rather weak, it’s weaker for long wavelength light (red light) than it is for short wavelength light (violet light). As a result, blue and violet lights are scattered more than red light; making the sky appear blue and the sun and moon appear red, particularly when they are low on the horizon and most of their blue light is scattered away before it reaches your eyes. When there is extra dust in the air, such as after a volcanic eruption, Rayleigh scattering is enhanced and the red sunsets are particularly intense.

Is it harmful for children to sit too close to microwave ovens? Is it possible t…

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Is it harmful for children to sit too close to microwave ovens? Is it possible to get “burned” opening the microwave oven during a cycle or too soon after a cycle? I realize the oven shuts off, but is there residual radiation? — C

As long as the microwave oven hasn’t been damaged and doesn’t leak excessive microwaves, there should be no harm in having children sit near it. I wouldn’t hold my face right up against the door edges because that would be asking for trouble with leakage, but it’s extremely unlikely that even doing that once in a while would cause injury.

As for being injured by microwave radiation after the cycle has stopped, that’s essentially impossible. As soon as the high voltage disappears from the magnetron tube and it stops emitting microwaves, the microwaves in the cooking chamber begin to diminish. Even if they bounce 1000 times off the metal walls of the chamber before they’re absorbed by those walls or the food in the microwave, that will only take about 2 millionths of a second. You can’t open the door fast enough to let them out before they’re already gone.

Please explain the “Wagon Wheel Effect.” How can the wheel appear to move forw…

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Please explain the “Wagon Wheel Effect.” How can the wheel appear to move forward, then backward, then stop, just by viewing it differently? — J, Davenport, IA

This effect is the result of viewing a series of stop-action frames in rapid sequence as a movie or video. Even though a wagon wheel is turning forward, its orientation during sequential frames of a movie may make it appear to be stopped or turning backward. For example, if the wagon wheel completes exactly one full turn between each frame of the movie, the wheel will appear to be stopped—its orientation in each frame will be the same. If it completes slightly less than one full turn between each frame, it will appear to be turning backward! As you can see, a tiny change in wheel rotation rate, from slightly more than one full turn per frame to slightly less than one full turn per frame, is enough to make the wheel appear to switch from turning forward, to stopped, to turning backward. So it’s no wonder that the wheels appear to change speeds abruptly from no apparent reason.

How do I make my own satellite descrambler/decoder?

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How do I make my own satellite descrambler/decoder?

Even if I knew, I’m sure that I’d get in trouble for telling. The encoding schemes are proprietary information and not available to the general public. To my knowledge, most of the descrambling/decoding in a satellite receiver is done by custom integrated circuits that are extremely difficult to reverse engineer (i.e., to open up, examine, and duplicate) so that pirating satellite signals is nearly impossible without insider information.

Is there a homing device small enough to fit onto or inside a pc laptop? How doe…

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Is there a homing device small enough to fit onto or inside a pc laptop? How does a homing device work?

There are homing devices small enough to fit on bugs, so there should be no problem fitting one on or into a laptop. A homing device is simply a radio transmitter and, while it has recently become possible to build a homing device that actually knows where it is and can tell you via its transmission, the techniques involved in locating most normal homing devices are those of trying to find the source of a radio transmission. Using directional receiving antennas and studying the transmission from several locations, you can figure out where the transmission is coming from.

How do electric/magnetic linear drives work?

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How do electric/magnetic linear drives work?

Linear electric motors are very much like rotary electric motors—they use the forces between magnetic poles to push one object relative to another. But while a rotary motor uses these forces to twist a rotor around in a circle, a linear motor uses these forces to push a carriage along a track. Both the carriage and the track must contain magnets and at least some of these magnets must be electromagnets that can be turned on and off, or reversed. By timing the operations of the electromagnets properly, the linear motor pushes or pulls the carriage along the track smoothly and continuously.

In making an electric generator, how do different aspects of the wire affect the…

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In making an electric generator, how do different aspects of the wire affect the total voltage and amperage? What are the effects of wire gauge, number of turns in the coils, and whether the magnets move past the coils or the coils past the magnets? — BLM, Houston, TX

First, it doesn’t matter when the magnet moves past the coils or the coils past the magnet; a generator will work the same way in either case. The voltage produced by the generator is determined by the number of turns in its coils, the strength of its magnet, and the rate at which its magnet turns. The more turns in the coils, the more work the generator does on each charge that passes through those coils and the more voltage the charges have when they leave the generator. The current that the generator can handle is limited by the power of its engine and by the wire’s ability to handle the current without wasting too much power. In general, a generator’s wire gauge is chosen to minimize power loss while keeping the coils reasonably small and light. If you try to send too much current through the generator, its engine may stall or its wires may overheat.

How would you construct and wire a battery recharger using solar panels as a vol…

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How would you construct and wire a battery recharger using solar panels as a voltage source? — JW, Kingston, Ontario

First, you would need to put enough solar panels in series to develop a voltage greater than that of your battery. For example, to recharge a 1.5 volt battery, you would probably have to attach three or four simple solar cells in series because each one only provides a current passing through it with about 0.5 volts of voltage rise. Having assembled enough solar cells, you should then attach the positive output terminal of the solar cell chain to the positive terminal of your battery and attach the negative output terminal of the solar cell chain to the negative terminal of your battery. When you put the solar cells in the light, they will begin to push electric current backward through the battery and the battery will recharge. Whenever you send current backward through a battery, its electrochemical reactions can run backward and it can recharge to some extent. Unfortunately, some batteries recharge more effectively than others—the bad ones just turn the recharging energy into thermal energy. The only real subtlety in this business is in stopping the charging when the battery is fully recharged. You should check the battery voltage periodically and when it’s close to the voltage of a new battery, it probably can’t take any more charging.

How do you make solar cells?

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How do you make solar cells? — BP

Solar cells are made in the same way that semiconductor diodes are made. Two different types of semiconductor, p-type and n-type, are joined together to form a diode—a one-way device for electric current. When light energy is absorbed in the n-type portion of the diode, it can propel an electron across the p-n junction between the materials and into the p-type material. Since the electron can’t return across the p-n junction to its original location, it must flow through an external circuit to get back. Since it obtains energy from the light that sent it across the junction, the electron can provide that energy to the circuit. The solar cell is thus a source of electric power.

A charge coupled device converts light (photonic energy) into electric energy. W…

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A charge coupled device converts light (photonic energy) into electric energy. What is the underlying mechanism that makes this happen? — PM, Belfast, Ireland

As in any photoelectric cell, the energy from a single particle of light—a photon—is used to raise the energy of an electron in a diode and to propel that electron from one side of the diode to the other. In this process, the light energy is partly converted to electrostatic potential energy and partly to thermal energy. Since a diode only carries current in one direction, the electron is unable to return to its original side. In a photoelectric cell, the electron flows through a circuit to return to the other side of the diode and provides energy to that circuit. In a charge coupled device, a complicated charge shifting system transfers the electrons to a detector that registers how much light was absorbed.