. There is a debate amongst the teachers in our school as to what are the three primary colors. Some say Red, Green, and Blue, others say Red, Yellow, and Blue. Do you have an explanation? — RS, Farmington Hills, MI
The true primary colors of light are Red, Green, and Blue. This empirical result is determined by physiological characteristics of the three types of color sensitive cells in our eyes. These cells are known as cone cells and are most sensitive to red light, green light, and blue light respectively. Light that falls in between those wavelength ranges stimulate the three groups of cells to various extents and our brains use their relative stimulations to assign a color to the light we're seeing. For example, when you look at yellow light, the red sensitive and green sensitive cone cells are stimulated about equally and your brain interprets this result as yellow. When you look at an equal mixture of red light and green light, the red sensitive and green sensitive cells are again stimulated about equally and your brain again interprets this result as yellow. Thus you can't tell the difference between true yellow light and an equal mixture of red light and green light. That's how a television tricks your eyes into seeing all colors. If you look closely at a color television screen, you'll see tiny dots of red, green, and blue light. But when you back up, you begin to see a broad range of colors. The television is mixing the three primary colors of light to make you see all the other colors.
Incidentally, the three primary colors of pigment are yellow, cyan, and magenta. Yellow pigment absorbs blue light, cyan pigment absorbs red light, and magenta pigment absorbs green light. When exposed to white light, a mixture of these three pigments controls the mixture of the reflected lights (red, green, and blue) and thus can make you see any possible color.
. Is there an effective shield for the EMF generated from mercury vapor ballasts? — CS, Washington, DC
An electric field can always been shielded by encasing its source in a grounded conducting shell. Electrically charged particles in the shell will naturally rearrange themselves in such a way as to cancel the electric fields outside the shell. But magnetic fields are harder to shield, particularly if they don't change very rapidly with time. The difficulty with shielding magnetic fields comes from the apparent absence of isolated magnetic poles in our universe—there is no equivalent of electrically charged particles in the case of magnetism. As a result, the only way to shield magnetic fields is to take advantage of the connections between electric and magnetic fields.
Because changing magnetic fields are always accompanied by electric fields, the two can be reflected as a pair by highly conducting surfaces or absorbed by poorly conducting surfaces. In these cases, the electric fields push and pull on electric charges in the surfaces and it is through these electric fields that the magnetic fields are reflected or absorbed. However, this effect works much better at high frequencies than at low frequencies, where very thick materials are required. Appliances that operate from the AC power line have magnetic fields that change rather slowly with time (only 120 reversals per second or 60 full cycles of reversal each second) and that are extremely hard to shield with conducting material. Instead, their magnetic fields have to be trapped in special magnetic materials that draw in magnetic flux lines and keep them from emerging into the surrounding space. One of the most effective magnetic shield materials is called "mu metal", a nickel alloy that's like a sponge for magnetic flux lines. Since it also conducts electricity pretty well, it is an effective shield for electric fields. So if you wrap your mercury vapor ballasts in mu metal, there would be almost no electric or magnetic fields detectable outside of the mu metal surface.
. Why do neutrinos only spin left? — BA, Fairbury, IL
The absence of right-handed neutrinos is simply a feature of our universe and I don't believe anyone has a good explanation for their absence. Actually, it's still possible that right-handed neutrinos exist, but if they do exist, then they don't interact with other matter by way of any known force other than gravity. Even left-handed neutrinos barely interact with matter—they experience only gravity and the weak force, and usually pass through the entire earth without being absorbed. It could be that right-handed neutrinos are also present but that they don't even experience the weak force.
. 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.
. 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.
. 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.
. 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 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.
. 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 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.