Why does light travel slower in some media than in a vacuum? For example, in gla…

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Why does light travel slower in some media than in a vacuum? For example, in glass or other transparent media, visible light is not absorbed and yet it slows down. What’s going on? — FH, Waltham, MA

When a light wave enters matter, the light wave’s electric field causes charged particles in the matter to accelerate back and forth. That’s because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don’t retain the light’s energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they reemit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then reemitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened—the light wave has been delayed ever so slightly. This absorption and reemission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Actually, higher frequency violet light is slowed more than lower frequency red light because violet light is more effectively absorbed and reemitted by the atoms in most transparent materials. That’s because when a high frequency light wave encounters the electrons in an atom, the jiggling motion is so rapid and the electrons’ motions are so small that the electrons never reach the boundaries of the atom. As a result, those electrons are able to jiggle back and forth as though they were free electrons and they do a good job of slowing the light wave down. But when a low frequency light wave encounters the electrons in an atom, the jiggling motion is slower and the electrons’ motions are so large that they quickly reach the boundaries of the atom. As a result, those electrons aren’t able to jiggle back and forth as far as they should and they don’t slow the light wave down as well.

What is infrared light?

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What is infrared light? — AC, Teaneck, NJ

Infrared, visible, and ultraviolet light are all electromagnetic waves. However these waves differ in both their wavelengths (the distances between adjacent maximums in their electric fields) and in their frequencies (the number of electric field maximums that pass by a specific point in space each second). Infrared light has longer wavelengths and lower frequencies than visible light, while ultraviolet light has shorter wavelengths and higher frequencies than visible light. We can’t see infrared or ultraviolet lights because the cells of retinas aren’t sensitive to these lights. Nonetheless, we can often tell when those lights are present—we may feel infrared light as heat on our skins and we may find ourselves sunburned by ultraviolet light.

I know that an electromagnetic wave cannot pass through the holes in a metal cag…

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I know that an electromagnetic wave cannot pass through the holes in a metal cage (a Faraday cage) if those holes are significantly smaller than the wavelength of the wave. But what if it is just a constant electric field? What determines the hole size now? — KBH, Logan, Utah

If the electric field isn’t changing with time, then it can’t enter a metal cage no matter how large the cage’s holes are. In effect, the constant electric field has an infinite wavelength and can’t propagate through holes of any finite size. However, the holes don’t stop an electromagnetic wave instantly—the wave does penetrate a short distance into the cage before it dwindles to insignificance. The distance over which the wave diminishes by a factor of about 3 is roughly the size of the hole through which it is trying to pass. So if your Faraday cage has holes that are 1 centimeter in diameter, the constant electric field will take several centimeters to diminish to nearly zero. If the holes are much larger than that, the electric field will penetrate far into the cage and the cage will only be an effective shield if it is extremely large. To avoid having to use a very large cage, it’s better to use small holes.

How do microwave ovens affect people fitted with pacemakers?

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How do microwave ovens affect people fitted with pacemakers? — W

If a microwave oven doesn’t leak microwaves, then it won’t affect such people at all. However, if microwaves do leak from a particular microwave oven, they will cause undesirable currents to flow in the electric leads of the pacemaker. That’s because a microwave consists of electric and magnetic fields, and an electric field exerts forces on charged particles. The mobile charged particles in the pacemaker’s electric wiring will experience these forces as the microwave encounters them and they will move back and forth with the microwave’s fluctuating electric field. The pacemaker’s wiring isn’t meant to carry these unexpected current flows, and the pacemaker and/or the person attached to it may experience unpleasant effects. While such problems are very unlikely, it makes sense to warn pacemaker users whenever a microwave oven is in use.

Do hand carried microwave heaters exist or must the microwaves always be enclose…

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Do hand carried microwave heaters exist or must the microwaves always be enclosed, as they are in a microwave oven? — AL, Umea, Sweden

My understanding is that there are microwave heating systems that are not enclosed and that are used in medical therapies to provide deep warming to injured tissues in medical patients. But apart from such devices, I’ve never heard of unenclosed microwave heaters. That’s because such heaters would be dangerous, since a user would be exposed to the heating effects of the microwaves. To keep the microwave heating under control, microwave ovens always carefully enclose the microwaves in a metal cooking chamber from which they can’t escape.

I’ve heard the reason an antenna, such as the one on your car, is so long is bec…

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I’ve heard the reason an antenna, such as the one on your car, is so long is because it needs to be large enough for the long radio waves to pass into it. Is this true? Why are antennas for radio stations so tall and slender? — LW, Blacksburg, VA

A vertical pole radio antenna receives a radio wave by allowing that wave to push electric charges up and down the antenna. The radio senses this moving charge and is thus aware of the passing radio wave. The ideal length of a vertical receiving antenna is a quarter of the wavelength of the radio wave it’s trying to receive—in which case, charge that the radio wave’s electric field pushes up and down the antenna has just enough time to reach the end of the antenna before it has to reverse directions.

The waves used for standard AM radio transmissions have very long wavelengths—typically 300 meters—so that they require vertical pole antennas that are about 75 meters long for optimal reception. An antenna of that length is also optimal for radio transmission, which is why the antennas of AM radio stations are so long and slender. However, because such long antennas are inconvenient for most AM receivers, most AM receivers use small magnetic antennas. A magnetic antenna is a device containing an iron-like material called ferrite that draws in magnetic flux lines like a sponge. A coil of wire is wound around this ferrite so that as the magnetic flux lines of a passing radio wave enter the ferrite, they induces electric currents into the coil of wire. This coil then acts as the antenna.

But the waves used in FM radio transmission have much shorter wavelengths—typically 3 meters—so that antennas of about 75 centimeters are all that’s needed. The vertical pole radio antenna on your car is designed to receive these FM waves. The antennas of FM radio stations are also rather short, but they are usually mounted high up on a pole so that the whole structure looks like an AM radio antenna. However, if you look near the top of an FM radio tower, you’ll see the actual FM antenna as a much smaller structure.

What kind of tape recorders are the best: cassette recorders or the ones with bi…

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What kind of tape recorders are the best: cassette recorders or the ones with bigger spools? — HB, Stde, Sweden

The audio quality of analog tape recording improves as the tape moves faster past the recording and playback heads. That’s because the faster tape motion spreads out the magnetized regions of tape over greater distances on the tape’s surface. A cassette tape moves so slowly that oppositely magnetized regions are often bunched tightly together and they demagnetize one another. This demagnetization produces high-pitched noise in the recording. In contrast, a reel-to-reel tape that moves rapidly past the heads has magnetized regions that are widely spaced on the tape’s surface and that are much less susceptible to demagnetization and noise.

What is the formula for finding the power in an AC circuit?

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What is the formula for finding the power in an AC circuit?

If an appliance receiving power from an AC power source behaves as an electric resistor—meaning that the current passing through it is proportional to the voltage drop across it—then it’s easy to calculate the power being consumed by this appliance. You simply multiply the voltage drop across the appliance (measured in volts) by the current passing through the appliance (measured in amperes) to obtain the power (measured in watts). The voltage drop across the appliance indicates how much energy the appliance extracts from each unit of charge pass through it and the current passing through the appliance is the measure of how many units of charge are passing through the appliance each second. Thus the product of voltage drop times current gives the energy that the appliance extracts from the current each second, which is the power extracted by the appliance. On the other hand, if the appliance behaves like an inductor or capacitor—meaning that the current passing through it isn’t proportional to the voltage drop across it—it’s much harder to calculate the power that the appliance is consuming.

How does an electric welder work?

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How does an electric welder work? — JE

An electric welder sends an electric current through an ionized gas, forming a pattern of current flow through the gas that is known as an arc. The ionized gases in this arc consist of electrons that are negatively charged and atoms or molecules that have lost electrons to become positively charged. The electrons flow toward the positively charged metal at one end of the arc while the positively charged ion flow toward the negatively charged metal at the other end of the arc. As these charged particles move, they collide frequently with one another and with gas atoms or molecules along their paths, and they convert some of their electric energies into thermal energy. These collisions also produce additional ions. The enormous amounts of thermal energy produced by collisions as the charged particles flow through the arc melts the metals at the ends of the arc so that these metals can be fused together.