Author: Lou Bloomfield

What does the SPF on sun screens mean? – RC

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What does the SPF on sun screens mean? – RC

Sunscreens contain pigments that absorb invisible ultraviolet radiation. While they appear clear and transmit visible light so that you can’t see them when they’re on your skin, sunscreens are almost opaque to ultraviolet light. A sunscreen’s SPF is related to the fraction of ultraviolet light that it absorbs. An SPF of 15 means that a normal layer of this sunscreen on your skin transmits only 1 part in 15 of the ultraviolet light that reaches it from the sun. An SPF of 40 means that a layer of this sunscreen transmits only 1 part in 40 of the ultraviolet light. The true transmission of the sunscreen depends somewhat on how you apply it and how much you apply, so these SPF ratings are only approximate. A sunscreen contains a mixture of dye molecules that transmit visible light but absorb ultraviolet light (and convert its the light’s energy into thermal energy). Most if not all of these dye molecules are artificial organic compounds that have been carefully selected to be non-toxic and non-irritating. The first popular sunscreen contained a compound called PABA that caused skin reactions in many people, but more recent dye choices are less likely to cause skin trouble.

What is a microwave and what does it do?

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What is a microwave and what does it do? — AH, Rochester, MN

A microwave is an electromagnetic wave with a frequency and a wavelength that are intermediate between those of a radio wave and those of light. An electromagnetic wave consists of both an electric field and a magnetic field. These two fields travel together in space and perpetually recreate one another as the wave moves forward at the speed of light. An electric field is a phenomenon that exerts forces on electric charges, while a magnetic field is a phenomenon that exerts forces on magnetic poles. Electric and magnetic fields are intimately connected, so that whenever an electric field changes, it creates a magnetic field and whenever a magnetic field changes, it creates an electric field. By combining a changing electric field and a changing magnetic field, the electromagnetic wave uses their abilities to create one another to form a self-perpetuating entity—the wave’s changing electric field creates its changing magnetic field and its changing magnetic field creates its changing electric field.

If you were to freeze an electromagnetic wave at one instant and look at its structure in space, you would find that its electric and magnetic fields had a periodic spatial structure. Just as a water wave has crests and troughs, an electromagnetic wave has spatial fluctuations in its two fields. The distance between adjacent “crests” in either one of these fields is that wave’s wavelength. Different types of electromagnetic waves have different wavelengths. Radio waves have long wavelengths that range from about 1 meter to hundreds or even thousands of meters and visible light has short wavelengths that range from about 400 billionths of a meter (400 nanometers) to about 750 billionths of a meter (750 nanometers). Microwaves are those electromagnetic waves with wavelengths between 1 millimeter and 1 meter. The microwaves used in microwave cooking have wavelengths of 12.2 centimeters.

Microwaves are often used to carry information in satellite communication and telephone microwave links. Whenever you see a dish antenna (a satellite dish or a communication link dish on a building or tower), you are looking at a microwave system. Astronomers use radio telescopes to look at microwave emissions from celestial objects. Radar bounces microwaves from objects to determine where they are or how fast they’re moving. And microwave ovens use microwaves to add thermal energy to water molecules in order to heat food.

Why does white noise cancel out the wide range of frequencies in the real world?…

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Why does white noise cancel out the wide range of frequencies in the real world? What range of frequencies does this technology affect? Can you block out the low thud of a neighbor walking in the unit above yours? — EH, Chelmsford, MA

In the context of sound, a source of white noise emits random, non-repetitive sound waves that have equal acoustic powers at all frequencies. That means that the source emits the same amount of energy each second at each frequency, over the entire audible spectrum. What white noise does is to numb your hearing by creating a featureless, uniform background noise at every frequency you can hear. Since your sensitivity to sound volume is logarithmic, meaning that the acoustic power in a sound has to double before you notice that it’s substantially louder, this uniform background makes it extremely difficult for you to hear small sounds. Regardless of a small sound’s frequencies, the white noise is already exposing your ears to those frequencies and the small sound only makes a small change in the volumes of these frequencies. For an analogy, think about how much more you would notice a small blinking red light in the dark than in bright white sunlight. Similarly, white noise creates the acoustic equivalent of white illumination, making it hard for you to notice small noises that would be very easy to hear against complete silence. If the sounds your neighbor makes are small enough, this numbing effect should make them much less noticeable.

There are also much more sophisticated devices that really cancel noise out. However, these look like earphones and must be worn directly on your ears. These devices use microphones to measure the pressure fluctuations in the sounds and then cause the earphones to create exactly the opposite pressure fluctuations. With these noise cancellation devices properly adjusted, the air pressure fluctuations that are sound never reach your ears at all—they are simply cancelled away to nothing before they arrive.

How does a tsunami form and how far does it go when it hits land?

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How does a tsunami form and how far does it go when it hits land? — JM, Berkley, MI

A tsunami is simply a giant surface wave on water. Surface waves have several important characteristics, one of which is wavelength—that is, the distance between one crest and the next. The longer its wavelength, the faster a surface wave moves and also the deeper it extends below the surface of the water. In general a surface wave extends downward about one wavelength, so that if the crests are 100 meters apart, the wave is about 100 meters deep.

The wavelength of a tsunami is enormous—hundreds or even thousands of meters. As a result, a tsunami travels hundreds of kilometers per hour and extends downward deep into the ocean. Because it disturbs so much water, it carries a great deal of energy and it delivers this energy to the shore when it hits. Tsunamis are normally created by earthquakes or volcanic eruptions that sudden shift the supporting surfaces of a large amount of water. The water experiences a sudden impulse when the land or seabed shifts and a wave is emitted. You can launch a similar wave simply by shaking the end of a basin of water. But when a large region of land or seabed moves, the wave that’s launched has a very long wavelength and tremendous energy. This tsunami heads off with enormous speed until it encounters the gradual shallowing of a seashore. There it becomes deformed because the lack of water in front of it causes its crest to become incomplete. Eventually the tsunami breaks in churning surf. The height of this breaking wave crest and the distance it travels onto shore before it stops depends on the total energy of the tsunami, but heights of 10 or 20 meters are not uncommon. Such waves can travel hundreds of meters up a beach or oceanfront if the slope is sufficiently gradual.

How efficient are solar energy cells and windmills in producing energy for every…

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How efficient are solar energy cells and windmills in producing energy for everyday use? — JJ, San Antonio, TX

There are several ways to measure their efficiencies. One way is to compare the energy these devices extract from sunlight or from the wind to the electric energy they produce. By that measure, solar cells are roughly 15% efficient and windmills are roughly 50% efficient. However, you’re probably most interested in their cost efficiency—in how much power these devices can produce for a given operating cost. By that measure, both devices are somewhat more expensive to build and operate than conventional fossil-fuel power plants. As a result, the United States continues to rely on fossil-fuel plants because they cost less for each kilowatt-hour of electric energy produced. Nonetheless, solar cells are gradually becoming cheaper and they may become cost effective in the next decade or two. Windmills are already cost effective in some countries that rely entirely on imported fossil fuels. Denmark, for example, uses windmills extensively for electric power. While windmill power plants do exist in the United States, they are largely the results of regulation rather than market forces. But that, too, may change in the next decade or two.

Why does my voice sound different to me when I listen to a recording of myself?

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Why does my voice sound different to me when I listen to a recording of myself?

When you hear yourself speak directly, much of the sound that reaches your ears travels to them through the bones and tissues of your head. This type of sound conduction tends to emphasize the low frequencies in your voice so that your voice sounds lower to you than it does to other people. When you listen to a recording of your voice, you are hearing your voice as other people hear it, without the modifying effects of bone and tissue conduction. Everyone else listening to the tape thinks that your voice sounds normal but you think it sounds higher than normal.

How do radio waves transport energy?

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How do radio waves transport energy? — AD, Manaus City, Amazonia, Brazil

Radio waves consist of nothing more than electric and magnetic fields that are perpetually recreating one another as they travel through space at the speed of light. An electric field is a phenomenon that exerts forces on electric charges and a magnetic field is a phenomenon that exerts forces on magnetic poles. Both electric and magnetic fields contain energy because they are capable of doing work on and thus transferring energy to electric charges or magnetic poles that they encounter. In a radio wave, this energy or capacity to do work moves along with the fields at the speed of light. The radio transmitter uses electric power to create the radio wave and the radio wave delivers that power to the receiver. While most modern receivers use local electric power to amplify the information arriving in the radio wave, simple “crystal radios” are able to reproduce sound using on the power that is arriving in the radio wave itself.

How can we polarize a molecule?

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How can we polarize a molecule? — AD, Manaus City, Amazonia, Brazil

Some molecules, including water, are naturally polarized. This means that they have a positively charged end and a negatively charged end. But even normally non-polar molecules such as carbon dioxide can be polarized by exposing them to strong electric fields. Electric fields exert forces on electric charges and cause the electric charges in a molecule to rearrange—the positive charges in the molecule shift in one direction and the negative charges in that molecule shift in the other. As a result of this applied electric field, the molecule acquires a polar character—a negatively charged end and positively charge end. However, this polar character disappears as soon as the electric field is removed.

Why do the earth’s oceans appear blue to an observer on the moon?

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Why do the earth’s oceans appear blue to an observer on the moon?

The earth’s oceans and sky both appear blue to everyone who observes them. They do this because water absorbs blue light less strongly than it absorbs other colors. When ocean water is exposed to sunlight (white light), it absorbs most of the red light quickly and a good fraction of the green light. But the blue light penetrates to considerable depth in the water and there is a reasonable chance that this light will be scattered back upward to an observer on the shore, in the air, or even on the moon.

What’s the difference between fluorescent, phosphorescent, and triboluminescent?…

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What’s the difference between fluorescent, phosphorescent, and triboluminescent? – DS

Fluorescence is the prompt emission of light from an atom, molecule, or solid that has extra energy. For example, when some of the dyes used in modern swimwear and clothing are exposed to ultraviolet light, they absorb the light energy and promptly reemit part of that energy as visible light—typically brilliant greens and oranges. In contrast, phosphorescence is the delayed emission of light by an atom, molecule, or solid that has extra energy. Glow-in-the-dark objects are phosphorescent—they are able to store the extra energy they obtain during exposure to light for remarkably long times before they finally release that stored energy as visible light. Systems that exhibit phosphorescence rather than fluorescent are those that have special high-energy states that have enormous difficulty radiating away energy as light. Finally, triboluminescence is the emission of light from a surface experiencing sliding friction. Since sliding friction introduces energy into the surfaces that are sliding across one another, it’s possible for that energy to be emitted as light.