Although that sounds like a simple question, it has a complicated answer. Gravity does affect light, but it doesn't affect light's speed. In empty space, light is always observed to travel at "The Speed of Light." But that remark hides a remarkable result: although two different observers will agree on how fast light is traveling, they may disagree in their perceptions of space and time.
When those observers are in motion relative to one another, they'll certainly disagree about the time and distance separating two events (say, two firecrackers exploding at separate locations). For modest relative velocities, their disagreement will be too small to notice. But as their relative motion increases, that disagreement will become substantial. That is one of the key insights of Einstein's special theory of relativity.
But even when two observers are not moving relative to one another, gravity can cause them to disagree about the time and distance separating two events. When those observers are in different gravitational circumstances, they'll perceive space and time differently. That effect is one of the key insights of Einstein's general theory of relativity.
Here is a case in point: suppose two observers are in separate spacecraft, hovering motionless relative to the sun, and one observer is much closer to the sun than the other. The closer observer has a laser pointer that emits a green beam toward the farther observer. Both observers will see the light pass by and measure its speed. They'll agree that the light is traveling at "The Speed of Light". But they will not agree on the exact frequency of the light. The farther observer will see the light as slightly lower in frequency (redder) than the closer observer. Similarly, if the farther observer sends a laser pointer beam toward the closer observer, the closer observer will see the light as slightly higher in frequency (bluer) than the farther observer.
How can these two observers agree on the speed of the beams but disagree on their frequencies (and colors)? They perceive space and time differently! Time is actually passing more slowly for the closer observer than for the farther observer. If they look carefully at each others' watches, the farther observer will see the closer observer's watch running slow and the closer observer will see the farther observer's watch running fast. The closer observer is actually aging slightly more slowly than the farther observer.
These effects are usually very subtle and difficult to measure, but they're real. The global positioning system relies on ultra-precise clocks that are carried around the earth in satellites. Those satellites move very fast relative to us and they are farther from the earth's center and its gravity than we are. Both difference affect how time passes for those satellites and the engineers who designed and operate the global positioning system have to make corrections for the time-space effects of special and general relativity.
The speed of light in vacuum, as denoted by the letter c, is truly a constant of nature and one of its most influential constant at that. Even if light didn't exist, the speed of light in vacuum would. It is a key component of the relationship between space and time known as special relativity.
But while the speed of light in vacuum is a constant, the speed of light in matter isn't. Light is an electromagnetic wave and consists of electric and magnetic fields. Electric fields push on electric charge and matter contains electric charges, so light and matter interact. That interaction normally slows light down; the light gets delayed by the process of shaking the electric charges. In air, this slowing effect is tiny, less than 1 part in a thousand. In glass, plastic, or water, light is slowed by about 30 or 40%. In diamond, the interaction is strong enough to slow light by 60%. In silicon solar cells, light is slowed by 70%. And so it goes.
To really slow light down, however, you need to choose a specific frequency of light and let it interact with a material that is resonant with that light. Because a resonant material responds extremely strongly to the light's electric field, it delays the light by an enormous amount. And by choosing just the right wavelength of light to match a particular collection of resonant atoms, Lene Hau and her colleagues managed to bring light essentially to a halt. The light lingers nearly forever with the atoms in their apparatus and it barely makes any headway.
Yes, you can tell how fully you have consolidated a powder by the extent to which it scatters light. The more perfect the packing, the more transparent the powder becomes. It's a matter of homogeneity: the more perfect the packing, the more homogeneous the material and the easier it is for light to travel straight through it.
To understand why light scatter depends on homogeneity, consider what happens when light pass through clear particles. Even though they are clear, light still interacts with them, as evidenced by rainbows, clouds, and even the blue sky. How best to think about that interaction depends on the size of the particles. If the particles are large, like smooth beads of glass or plastic, then they exhibit the familiar refraction and reflection effects of window panes and lenses. If the particles are small, like air molecules and tiny water droplets, then they exhibit a more antenna-like interaction with light. In effect, those tiny particles occasionally absorb and reemit the light waves, particularly at the short-wavelength (i.e., blue) end of the light spectrum.
Both types of interactions are quite familiar to us. Large particles scatter light about without any color bias and exhibit a white appearance. The more surface area a collection of particles has, the more light that collection scatters. For example, a large ice crystal is clear but crushed ice or snow is white. Similarly, a bowl of water is clear but a mist of water droplets is white. Lastly, a bowl of air is clear, but a froth of air bubbles in water is white. As you can see, the transparent particles don't have to be solids or liquids to scatter light, they can even be gases!
On the other hand, truly tiny particles scatter light about according to wavelength and color. In most cases, shorter-wavelength (blue) light scatters more than longer-wavelength (red) light. That effect, known as Rayleigh scattering, is responsible for the blue sky and the red sunset.
In a nutshell then, large transparent particles appear white and tiny transparent particles appear colored (typically bluish). And the more particles there are, the more light is scattered.
Returning to your question, a loose powder of transparent particles scatters light like crazy and appears white or possible colored, depending on particle size. As you pack the powder more and more tightly together, its surfaces join together and it starts to lose the ability to scatter light; it becomes less white and more translucent. When the consolidation is almost complete, the material acquires a slightly hazy look due to scattering by the occasional voids left inside the otherwise transparent material. Finally, when the material is fully consolidated and there is no internal surface left in the powder, it is homogeneous and clear. So sending light through a packed transparent powder and measuring the amount and color of the scattered light tells you a lot about how well consolidated that powder is.
As I discussed previously, the sky is blue because tiny particles in the atmosphere (dust, clumps of air molecules, microscopic water droplets) are better at deflecting shorter wavelength blue light than they are at deflecting longer wavelength red light. As sunlight passes through the atmosphere, enough blue light is deflected (or more technically Rayleigh scattered) by these particles to give the atmosphere an overall blue glow. The sun itself is slightly reddened by this process because a fraction of its blue light is deflected away before it reaches our eyes.
But at sunrise and sunset, sunlight enters our atmosphere at a shallow angle and travels a long distance before reaching our eyes. During this long passage, most of the blue light is deflected away and virtually all that we see coming to us from the sun is its red and orange wavelengths. The missing blue light illuminates the skies far to our east during sunrise and to our west during sunset. When the loss of blue light is extreme enough, as it is after a volcanic eruption, so little blue light may reach your location at times that even the sky itself appears deep red. The particles in air aren't good at deflecting red wavelengths, but if that's all the light there is they will give the sky a dim, red glow.
The foam consists of tiny air bubbles surrounded by very thin films of soap and water. When light enters the foam, it experiences partial reflections from every film surface it enters or exits. That is because light undergoes a partial reflection whenever it changes speed (hence the reflections from windows) and the speed of light in soapy water is about 30% less than the speed of light in air. Although only about 4% of the light reflects at each entry or exit surface, the foam contains so many films that very little light makes it through unscathed. Instead, virtually all of the light reflects from film surfaces and often does so repeatedly. Since the surfaces are curved, there is no one special direction for the reflections and the reflected light is scattered everywhere. And while an individual soap film may exhibit colors because of interference between reflections from its two surfaces, these interference effects average away to nothing in the dense foam. Overall, the foam appears white—it scatters light evenly, without any preference for a particular color or direction. White reflections appear whenever light encounters a dense collection of unoriented transparent particles (e.g. sugar, salt, clouds, sand, and the white pigment particles in paint).
As for the fact that even colored soaps create only white foam, that's related to the amount of dye in the soaps. It doesn't take much dye to give bulk soap its color. Since light often travels deep into a solid or liquid soap before reflecting back to our eyes, even a modest amount of dye will selectively absorb enough light to color the reflection. But the foam reflects light so effectively with so little soap that the light doesn't encounter much dye before leaving the lather. The reflection remains white. To produce a colored foam, you would have to add so much dye to the soap that you'd probably end up with colored hands as well.
Most objects make no light of their own and are visible only because they reflect some of the light that strikes them. Without sunlight (or any other light source), these passive objects would appear black. Black is what we "see" when there is no light reaching our eyes from a particular direction. The only objects we would see would be those that made their own light and sent it toward our eyes.
The fact that we see mostly reflected light makes for some interesting experiments. A red object selectively reflects only red light; a blue object reflects only blue light; a green object reflects only green light. But what happens if you illuminate a red object with only blue light? The answer is that the object appears black! Since it is only able to reflect red light, the blue light that illuminates it is absorbed and nothing comes out for us to see. That's why lighting is so important to art. As you change the illumination in an art gallery, you change the variety of lighting colors that are available for reflection. Even the change from incandescent lighting to fluorescent lighting can dramatically change the look of a painting or a person's face. That's why some makeup mirrors have dual illumination: incandescent and fluorescent.
The one exception to this rule that objects only reflect the light that strikes them is fluorescent objects. These objects absorb the light that strikes them and then emit new light at new colors. For example, most fluorescent cards or pens will absorb blue light and then emit green, orange, or red light. Try exposing a mixture of artwork and fluorescent objects to blue light. The artwork will appear blue and black: blue wherever the art is blue and black wherever the art is either red, green, or black. But the fluorescent objects will display a richer variety of colors because those objects can synthesize their own light colors.
Ozone is an unstable molecule that consists of three oxygen atoms rather than then usual two. Because of its added complexity, an ozone molecule can interact with a broader range of light wavelengths and has the wonderful ability to absorb harmful ultraviolet light. The presence of ozone molecules in our upper atmosphere makes life on earth possible.
However, because ozone molecules are chemically unstable, they can be depleted by contaminants in the air. Ozone molecules react with many other molecules or molecular fragments, making ozone useful as a bleach and a disinfectant. Molecules containing chlorine atoms are particularly destructive of ozone because a single chlorine atom can facilitate the destruction of many ozone molecules through a chlorine recycling process.
In contrast, nitrogen molecules are extremely stable. They are so stable that there are only a few biological systems that are capable of separating the two nitrogen atoms in a nitrogen molecule in order to create organic nitrogen compounds. Without these nitrogen-fixing organisms, life wouldn't exist here. Because nitrogen molecules are nearly unbreakable, they survive virtually any amount or type of chemical contamination.
When a light wave passes through matter, the charged particles in that matter do respond—the light wave contains an electric field that pushes on electrically charged particles. But how a particular charged particle responds to the light wave depends on the frequency of the light wave and on the quantum states available to the charged particle. While the charged particle will begin to vibrate back and forth at the light wave's frequency and will begin to take energy from the light wave, the charged particle can only retain this energy permanently if doing so will promote it to another permanent quantum state. Since light energy comes in discrete quanta known as photons and the energy of a photon depends on the light's frequency, it's quite possible that the charged particle will be unable to absorb the light permanently. In that case, the charged particle will soon reemit the light.
In effect, the charged particle "plays" with the photon of light, trying to see if it can absorb that photon. As it plays, the charged particle begins to shift into a new quantum state—a "virtual" state. This virtual state may or may not be permanently allowed. If it is, it's called a real state and the charged particle may remain in it indefinitely. In that case, the charged particle can truly absorb the photon and may never reemit it at all. But if the virtual state turns out not to be a permanently allowed quantum state, the charged particle can't remain in it long and must quickly return to its original state. In doing so, this charged particle reemits the photon it was playing with. The closer the photon is to one that it can absorb permanently, meaning the closer the virtual quantum state is to one of the real quantum states, the longer the charged particle can play with the photon before recognizing that it must give the photon up.
A colored material is one in which the charged particles can permanently absorb certain photons of visible light. Because this material only absorbs certain photons of light, it separates the components of white light and gives that material a colored appearance.
A transparent material is one in which the charged particles can't permanently absorb any photons of visible light. While these charged particles all try to absorb the visible light photons, they find that there are no permanent quantum states available to them when they do. Instead, they play with the photons briefly and then let them continue on their way. This playing process slows the light down. In general blue light slows down more than red light in a transparent material because blue light photons contain more energy than red light photons. The charged particles in the transparent material do have real permanent states available to them, but to reach those states, the charged particles would have to absorb high-energy photons of ultraviolet light. While blue photons don't have as much energy as ultraviolet photons, they have more energy than red photons do. As a result, the charged particles in a transparent material can play with a blue photon longer than they can play with a red photon—the virtual state produced by a blue photon is closer to the real states than is the virtual state produced by a red photon. Because of this effect, the speed at which blue light passes through a transparent material is significantly less than the speed at which red light passes through that material.
Finally, about quantum states: you can think of the real states of one of these charged particles the way you think about the possible pitches of a guitar string. While you can jiggle the guitar string back and forth at any frequency you like with your fingers, it will only vibrate naturally at certain specific frequencies. You can hear these frequencies by plucking the string. If you whistle at the string and choose one of these specific frequencies for your pitch, you can set the string vibrating. In effect, the string is absorbing the sound wave from your whistle. But if you whistle at some other frequency, the string will only play briefly with your sound wave and then send it on its way. The string playing with your sound waves is just like a charged particle in a transparent material playing with a light wave. The physics of these two situations is remarkably similar.
Apparently there are conditions in which green light from the sun is bent by the atmosphere so that it is visible first as the sun begins to rise above the horizon. Instead of seeing the yellow edge of the sun peaking up from behind the water or land, you see a green edge that lasts a second or two before being replaced by the usual yellow. This green flash is the result of refraction (bending of light) and dispersion (color-dependent light-speed) in air and is discussed in considerable detail at http://www.isc.tamu.edu/~astro/research/sandiego.html. According to the author of that site, Andrew Young, given a low enough horizon, which is the primary consideration, and clear air, which is also important, and a little optical aid, which helps a lot, one can certainly see green flashes at most sunsets.
When light passes into a material, it interacts primarily with the negatively charged electrons in that material. Since light consists in part of electric fields and electric fields push on charged particles, light pushes on electrons. If the electrons in a material can't move long distances and can't shift from one quantum state to another as the result of the light forces, then all that will happen to the light as it passes through the material is that it will be delayed and possibly redirected. But if the electrons in the material can move long distance or shift between states, then there is the chance that the light will be absorbed by the material and that the light energy will become some other type of energy inside the material.
Which of these possibilities occurs in a particular organic material depends on the precise structure of that material. Carbon atoms can be part of transparent organic materials, such as sugar, or of opaque organic materials, such as asphalt. The carbon atoms and their neighbors determine the behaviors of their electrons and these electrons in turn determine the optical properties of the materials.
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