. What is the purpose of the grid on the glass door of the microwave oven?
The metal grid reflects microwaves and keeps them inside the oven. Electromagnetic waves are unable to pass through holes in conducting materials if those holes are significantly smaller than their wavelengths. The wavelengths of visible light are very short, so light has no trouble passing through the holes in this grid. But the microwaves used in the oven have wavelengths of about 12.4 cm and are unable to propagate through the grid. Thus you can see the food cook while the microwaves are trapped inside the oven.
. At what angles do light rays reflect out of a prism? — BC, Farmersville, TX
It depends on the shape of the prism and the angle at which the light arrived at the prism. Whenever light's speed changes as it passes through a surface at an angle, the light bends. Since light travels faster in air than in glass (or plastic), it bends when it goes from air to glass or from glass to air. When light enters glass, it slows down and it bends toward the normal to the surface (toward the line that's at right angles to the surface). When light leaves glass, it speeds up and it bends away from the normal to the surface. To know exactly how far the light bends, you need to know how much the glass slows light (the glass's refractive index) and the angle at which the light encountered the glass surface (the angle of incidence). You can then use one of the basic laws of optics, Snell's law, to determine the angle at which the light continues through the glass. You can then do the same for the light's emergence from the glass and determine the angle at which it leaves.
. In a CD player, how is the digital optical signal transformed into an electrical signal? — IM, Oxford, UK
The ridges and flat regions on a compact disc's aluminum layer determine how laser light is reflected from that layer. As the disc turns and the player's laser scans across ridges and flat regions, the intensity of the reflected light fluctuates up and down. This reflected light is directed onto an array of silicon photodiodes that provide both the signals needed to keep the laser focused tightly on the aluminum layer and the signal that the player uses to recreate sound. The sound is encoded in the lengths of the ridges. A computer monitors the amount of light returning from the disc to determine how long each ridge is and how much spacing there is between it and the next ridge. The computer uses this information to obtain a series of 16 bit binary numbers for each of the two sound channels that are represented by an audio CD. A digital-to-analog converter uses these 16 bit numbers to produce currents that are eventually amplified and used to produce sound.
. How does hair spray work? — KC, IL
While I don't know exactly what chemicals are used in hairspray, the main constituents are almost certainly polymer molecules—otherwise known as plastics. In the container, these polymer molecules are dissolved in a volatile solvent such as an alcohol or water, and pressurized with a chemical such as propane or a hydrofluorocarbon. When you spray the mixture onto your hair, the solvent evaporates and leaves the polymer molecules clinging to the hairs. These molecules are very long chains of atoms that form a stiff web around each hair and stiffen it. In general, the characteristics of polymers change with temperature and chemical environment. The polymer used in hairspray should be in the "glassy" regime, meaning that its atoms and molecules are essentially immobile at room temperature. Once the solvent is gone, the web of polymer molecules on the hairs is stiff and keeps the hairs from changing shape. Before you panic at the idea of spraying plastic onto your hair, consider that starch is also a polymer, as is hair itself. So putting hairspray on your hair is no different from putting starch on clothes.
. How does a turbine flow meter work?
There are many different types of flow meters, some specialized to handling gases and others to handling liquids. In each case, a true flow meter transfers gas from its inlet to its outlet one unit of volume at a time and it measures how many of those volumes it transfers. There are also some flow rate
meters that measure how quickly a gas or liquid is flowing. These devices normally use of turbines to measure the speed of the passing fluid and measurements from these flow rate meters can be integrated over time to determine how much gas or liquid has passed through them. However, because flow rate meters don't measure each volume of gas directly, they aren't as accurate as true flow meters.
Let me assume that you want to know about a turbine flow meter for gas. The most common of these is a device that's half filled with liquid. The "turbine" is actually a set of blades that spin in a vertical plane and spend half their times immersed in the liquid. When one of the turning blades emerges from the liquid, the empty space that appears beneath it is allowed to fill with the gas being measured. This gas flows in from the meter's inlet. Soon another blade begins to emerge from the liquid and a volume of gas is then trapped between the first blade and the second blade. Once the blades have turned almost half a turn, the first one begins to submerge again in the liquid. The gas that was trapped between it and the next blade is then squeezed out from between those blades by the liquid and flows out the meter's outlet. A geared arrangement measures how many turns the blades make and therefore how many volumes of gas have been transferred from the meter's inlet to its outlet.
. Years ago I heard or read that some incandescent bulbs in Thomas Edison's house are still burning after being turned on back early in the 20th century. Is this true? What are they made of?
From comments that I've received over the web, this story is apparently true. However those bulbs must be operating at reduced power levels and are glowing dimly as a result. There is no magic filament material that can operate indefinitely at yellow-white heat. The life of a filament is determined by how quickly its atoms evaporate (actually sublime) from its surface. Modern tungsten filaments operate at about 2500° C. At that temperature, the filament loses atoms slowly enough that it lives for about 1000 hours. If you were to operate the filament several hundred degrees colder, it would live much, much longer but it wouldn't emit nearly as much light and what light it did emit would be relatively reddish. The design of incandescent bulbs is a trade-off of energy efficiency and operating life. Long-life bulbs are substantially less energy efficient than normal bulbs—you don't have to replace them as often but they cost more to operate. Getting back to Edison's bulbs: they can only live long lives by operating at less than normal temperatures. In that case, they may live a hundred years but have very poor energy efficiencies.
. Does hot water really freeze quicker than water at room temperature? — MH, Dallas, TX
In most cases the answer is no. All things being equal, the room temperature water will have a head start and will freeze first. The hot water must first cool down to room temperature and then it will simply follow the behavior of the room temperature water. However, in the special case where the water is held in an insulated container that's open at the top, it's possible for hot water to freeze faster. That's because evaporation of water molecules from the exposed surface of the hot water makes an important contribution to the cooling process in that case and a significant fraction of the water molecules will have left the container by the time it reaches room temperature. Since it takes less time to freeze a smaller quantity of water, the container of hot water can freeze before the container of room temperature water. However, there will be less ice in the container that was once filled with hot water.
There is another interesting effect that occurs when freezing hot water. If you boil water, you will drive most of the dissolved gases out of it. You see these gases emerge as bubbles on the sides of a pot as the water heats up when you put the pot on the stove. If you freeze boiled water, it will probably freeze slightly faster than unboiled water. That's because the dissolved gases also come out of solution during the freezing process and these gases form bubbles in the ice. These bubbles slow the flow of heat through the ice and delay the freezing of its center. Thus, while room temperature water will freeze quicker than hot water, previously boiled water that's now at room temperature will freeze even quicker than normal room temperature water. The boiled water will also form clearer ice cubes—they won't have any bubbles in them.
Finally, John Newell points out an interesting practical reason why hot water may sometimes freeze faster than cooler water in a household refrigerator—the temperatures of those refrigerators fluctuate because their thermostats have hysteresis. Once it has stopped operating, a refrigerator's compressor won't turn on again until the refrigerator temperature drifts upward significantly. If you put cool water in the refrigerator's freezing compartment, it may be quite a while before the compressor turns on and the refrigerator begins to pump heat out of the freezing compartment. But if you put hot water in the compartment, you may raise the temperature of the refrigerator enough to start the compressor, thus accelerating the freezing of the water.
. How can you make a makeshift water distiller with common materials? - VL
To distill water, you need to condense steam on a cold surface and collect the condensate. You could boil water in a teapot and allow the steam flowing out of its mouth to pass across the underside of a clean metal bowl full of ice water. The steam would condense as pure distilled water on the outside of the bowl. If you then placed a clean cup under the metal bowl, the distilled water would drip into that cup.
. Were microwaves invented for the microwave oven?
While microwaves were known long before anyone know how to produce them efficiently, they became important during World War II as the basis for radar. The ability to detect and locate enemy aircraft at long distances and at night was crucial to the defense of Allied cities during the war. The 1945 discovery that microwaves also cooked food was an accidental offshoot of radar development.
. Who invented the telephone dial or rotary portion of the telephone? — B, R, B, D, and S at Northlake Elementary, Richardson, TX
Let me start a little earlier, with the automatic telephone exchange: This exchange was invented in 1892 by Alman B. Strowger, an undertaker from Kansas, who first installed it in La Poste, Indiana. The system used electromagnetic relays to recognized a series of pulses and to make the appropriate connections between telephones. While the "Strowger system" remained in use until the advent of modern electronic switching systems, it was improved many times. The pulses that controlled this system were originally made with push buttons and one of the most important improvements was to replace the push buttons with a rotary dial that created the pulses automatically. However, the rotary dial wasn't so much invented as developed and I haven't found any record of the individuals who contributed to that development. No doubt it's a patented device and the patent record probably includes the names of the people responsible. If I can find that patent, I'll add it here.