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.

How much natural pressure is around us when we are on the ground? Does this pres…

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How much natural pressure is around us when we are on the ground? Does this pressure decrease in higher places? Why don’t people in aircraft explode because the pressure is lower?

Near sea level, the air around us has a pressure of about 100,000 newtons per square meter or 15 pounds per square inch. That means that each square meter of surface on your body is exposed to an inward force of 100,000 newtons or that each square inch of your body is exposed to an inward force of 15 pounds. Your body is thus exposed to enormous inward forces. However, you don’t notice these forces because your body is composed of solids and liquids that resist compression ferociously. To see that this is so, try to squeeze a sealed bottle of soda or to squash a coin by stepping on it. It’s very hard to shrink the volume of a solid or liquid by squeezing it.

The origin of the large pressure around us is the weight of the atmosphere overhead. The air near you is supporting the weight of several miles or kilometers of air overhead and the weight of this air is squeezing the air down here. When you ascend a mountain, the amount of air overhead decreases and so does the pressure of the air around you. Your body becomes less tightly squeezed by the air around it. However, you don’t explode because releasing the pressure on you doesn’t change your volume very much. Solids and liquids don’t expand very much when the pressure on them is released.

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.

I’ve seen tops that rest with their large parts down but that flip up onto their…

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I’ve seen tops that rest with their large parts down but that flip up onto their handles when you spin them. What is the reason that they have a different equilibrium when they are spinning versus when they are not? — CH, Renton, WA

While I’m not an expert on these “tipple tops,” I believe that I understand how they work. These tops have large round heads and look like wooden mushrooms. When you hold the handle (the mushroom’s stem) and spin it with its head down, it quickly flips over so that it spins on its handle. The flipping is caused by a torque that friction exerts on the top’s round head as the tops surface slides across the table. If the top were perfectly vertical as it spun on its head, friction between the top and the table would exert a torque (a twist) on the top that would simply slow the top’s rotation. But when the top isn’t perfectly vertical, the torque that friction exerts on it does more than slow its rotation. This torque also causes the top to precess (change its axis of rotation) in such a way that the top’s handle gradually becomes lower and the top’s head gradually becomes higher. Eventually, the top’s axis of rotation inverts completely so that it begins to rotate on its handle. Once that happens, the precession stops because the handle is too narrow for anything but the slowing effects. Only when the top stops spinning does it shift from this dynamically stable arrangement (handle down) to its statically stable arrangement (head down).

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.

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.

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.

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.

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 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.