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Sunlight
Page 5 of 9 (85 Questions and Answers)

485. Is it possible to create a "fog" in a small enclosed area without using dry ice or ultrasound?
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The two techniques you mention, dry ice and ultrasound, are both intended to make tiny droplets of water in the air, effectively producing an artificial cloud. While I can't think of any better ways to make such water droplets, I can think of ways to make fogs of other materials. Tiny particles of any clear material will work because what you are seeing is the random scattering of light as it's partially reflected from the front and back surfaces of clear particles. I'd suggest a chemical process that produces tiny clear particles. The easiest one I can think of is to place a dish of household ammonia (ammonium hydroxide—ammonia gas dissolved in water) and a dish of hydrochloric acid (hydrogen chloride gas dissolved in water, sold as muriatic acid by hardware stores) in your enclosed area. The two gases will diffuse throughout your enclosure and react to form tiny clear particles of ammonium chloride. The enclosure will fill with a dense white fog. The particles are so small, that they will remain in the air for a very long time, but they will eventually settle on surfaces and leave a white powdery residue. So, unlike a water fog, this chemical fog is a little messy. You shouldn't breathe the fog, either.

524. Does light have mass? If so, then how can it travel at the speed of light? Doesn't the mass of an object (particle) approach infinity as its velocity approaches the speed of light?
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Light has precisely zero mass and that makes all the difference. You're right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle's mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn't change. However, the relationship between the particle's energy and its momentum does change with speed and the particle's momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious "mass" to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle's speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its "mass." Not surprisingly, the limitless rises in energy, momentum, and "mass" prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can't be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can't travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.


537. Why do the earth's oceans appear blue to an observer on the moon?
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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.

542. What does the SPF on sun screens mean? - RC
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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.

562. At what angles do light rays reflect out of a prism? — BC, Farmersville, TX
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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.

573. How are light and sound the same? How are they different? — JS, Binghamton, NY
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There are so many answers to these questions that I'll have to pick and choose. For their similarities, I'll note that they're both disturbances that travel through space and that both have wavelengths and frequencies. Sound is a pressure disturbance in the air (or in another material) and consists of compressions and rarefactions that travel outward from their origin. The distance between adjacent regions of compression (or rarefaction) is the sound's wavelength and the number of compressed regions that pass by a particular point each second is the sound's frequency (or pitch). Light is an electromagnetic disturbance in space itself, although materials that are present in that space can alter its characteristics somewhat. It consists of electric and magnetic fields that travel outward as waves from their origin. The distance between adjacent regions of maximum electric field (or magnetic field) in one direction is the light's wavelength and the number of regions in which the electric field points maximally in a particular direction that pass by a particular point each second is the light's frequency (or color). I hope that you can see some of the similarities in these descriptions.

As for differences, sound is a longitudinal wave—meaning that the air involved in the pressure fluctuations moves back and forth in the direction of the wave's travel. Thus if sound is moving from left to right, the air is also fluctuating back and forth from left to right. In contrast, light is a transverse wave—meaning, that the electric and magnetic fields involved in the wave fluctuate back and forth at right angles to the direction of the wave's travel. Thus if light is moving from left to right, the electric and magnetic fields associated with it are fluctuating either up and down or toward you and away from you (or both). Another difference is that sound travels about 300 meters per second and its speed depends on the speed of the air through which it travels. Light, on the other hand, travels about 300,000 kilometers per second and its speed in vacuum (empty space) is absolutely constant. The speed of light is one of the fundamental constants of the universe.


585. Where does the white go when the snow melts? - JM
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To start with, light slows down when it moves from air to ice and speeds up when it moves from ice to air. That's because the electric charges in matter can delay a light wave and slow it down. Since electric charges are more concentrated in ice than they are in air, light travels more slowly in ice than it does in air. Next, some light reflects whenever light changes speed. That's why a glass windowpane reflects some light from both its front and back surfaces. Similarly, light reflects from each surface of an ice crystal. Finally, snow is a jumbled heap of ice crystals. These clear crystals partially reflect light in all different directions like billions of tiny mirrors. The result is a white appearance. You see this exact same effect when you look at white sand, granulated salt, granulated sugar, clouds, fog, white paint, and so on. Each of these materials consists of tiny, clear objects that partially reflect light in all directions. Since they reflect all colors of light equally and spread that light in all direction equally, they appear white.

When the snow melts and becomes water, it stops having all those tiny surfaces to partially reflect light. Instead, it has only its top surface and this surface continues to partially reflect light. That's why you see reflections in the top of a puddle.


594. How does a prism work? — RH, Louisville, KY
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When light enters a material such as glass, the light slows down. That's because the electric charges in the material delay a light wave by interacting with the wave's electric and magnetic fields. The higher the frequency of the light wave, the more it interacts with the charges in most materials and the more that light wave slows down. Thus high-frequency violet light slows more than low-frequency red light as the two enter a piece of glass.

Because of this slowing effect, light bends when it encounters a glass surface at an angle. The wave has a width and as it encounters the glass surface, one side of the wave reaches the glass before the other side of the wave. Since the side that arrives first also slows first, the whole wave bends so that it travels more directly into the glass. Since violet light slows more than red light, the violet light also bends more than the red light. The two colors thus follow different paths through the glass.

The same bending occurs in reverse when the light leaves the glass. Light speeds up as it leaves glass and again the violet light bends more than the red light. In a prism (or any carefully cut glass, crystal, or plastic), the colors of light bend differently at each surface and follow slightly different paths both in and out of the prism. The light rays then appear separately when they strike a surface outside the prism or when you look at those light rays with your eyes.


625. What material is used in glass to make it polarize light? — FG, Torrance, CA
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Actually, the polarizing material you are referring to is a plastic that has been impregnated with iodine atoms. The plastic, polyvinyl alcohol, is heated and stretched to align its long molecules in a particular direction. This plastic is then exposed to iodine, which binds to the long molecules and forms the equivalent of molecular wires along the direction of the aligned plastic molecules. These molecular wires absorb light that is polarized along them because the light's electric field points along its polarization direction and pushes electric charges wastefully along the iodine wires. This light is absorbed and its energy is converted to thermal energy, leaving only light with the other polarization.

636. Why is the sky blue? - Z
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As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That's because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

A reader (TAC) points out that the above explanation would seem to imply that the sky should appear violet, since violet light scatters more strongly than blue light. But the spectrum of sunlight peaks in the green—sunlight contains more green light than blue light and more blue light than violet light. The sky combines these two effects together (more green light but better scattering of violet light) and acquires an overall blue appearance.


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