How Everything Works
How Everything Works How Everything Works
 

QUESTIONS AND ANSWERS
 
Sunlight
Page 2 of 9 (85 Questions and Answers)

340. How do window tints (for your car windows) work? Are they just polarized materials?
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Some of them may be polarized materials, blocking horizontally polarized light, but most are simply absorbing materials that are embedded directly in the glass during its manufacture. Chemically tinted glass just darkens the sky be absorbing some of the light passing through the glass, regardless of polarization. It's not possible to chemically treat the glass to make it absorb only one polarization of light because that treatment would have to carefully align its molecules. In the plastic polarizing sheets, there is an alignment process (usually stretching in one direction) that lines up all the absorbing molecules.

341. How does light cancel in destructive interference?
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When two identical waves (usually two halves of the same wave) arrive together out of phase, the electric field in one wave (or half-wave) is up at the same moment that the electric field of the other wave (or half-wave) is down. These two electric fields add together and create a total electric field that is neither up nor down. An electric charge at this location in space will experience no forces so there is no electric field (one wave pushes that charge up while the other wave pushes that charge down). With no electric field around, there is no light to be absorbed. If two waves coming toward you interfere destructively, you will see no light. You might worry about conservation of energy; where did the light and its energy go? It went somewhere else. Any time there is destructive interference at one point in space, there will always be some other point in space at which there is constructive interference. Thus when you look at a soap film and see no red light, you can be sure that the red light has gone somewhere else. In the case of the soap film, when you see no red light in the reflection from the film, that red light has been transmitted by the film and is visible on the opposite side of the film.

342. How does light create heat?
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Actually, some light is heat. Heat is the energy that flows from one object to another because of a difference in their temperatures. The sun is hotter than you are so that it sends heat toward you. Sunlight is heat; it is the sun's heat being sent toward you as electromagnetic radiation. When it strikes the surface of your skin, this radiation is absorbed and becomes the more familiar form of heat: kinetic and potential energy in the atoms and molecules. From the surface of your skin, this heat flows inward to warm the rest of your body. Any material that absorbs light usually converts it to heat. The charged particles in that material move under the influence of the light's electric field and these moving charged particles transfer their energy here and there as heat.

343. How does suntan lotion work to prevent ultraviolet rays from damaging your skin?
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Suntan lotion (or rather sunscreen) is a chemical whose molecules absorb ultraviolet light and turn its energy into heat. Like fluorescent compounds, these molecules absorb ultraviolet light strongly. But unlike fluorescent compounds, the sunscreen molecules do not reemit any light. They convert all of the ultraviolet light energy into heat, which does no damage to your skin.

344. If white color has a reflection close to one, what role does shininess or dullness play?
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Just because two materials both reflect all of the light that strikes them doesn't mean that they look the same. When you send a flashlight beam at a white surface, you can see that reflected light from all directions. When you send the flashlight beam at a mirror surface, you can only see the reflected light from one particular angle. Both the white surface and the mirror surface reflect virtually all of the light that hits them. A shiny white surface is different from a dull white surface because a shiny white surface has a small amount of mirror character to it: you can see the whiteness from any direction but there is also a mirror aspect that you can only see from certain angles.

345. Is refraction the idea behind eyeglasses? If so, how?
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Yes, refraction is used in eye glasses. By carefully sculpting the front and back surfaces of a sheet of glass or plastic, the light passing through that sheet can be bent in remarkable ways. We will look at image formation in the section on Cameras.

346. What causes a magnifying glass within a ray of sun to burn such a small, specific spot? Is it the shape of the glass?
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The magnifying glass is a lens, a carefully shaped piece of glass that can refract sunlight to create an image. When you burn wood with a magnifying glass, you are creating an image of the sun on the wood. This tiny image, a circle that looks just like the sun itself, only much smaller, is so bright and contains so much thermal radiation that it overheats the wood that it strikes and causes that wood to burn.

347. What causes the colors in the aurora borealis?
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These colors come from the atomic fluorescence of particles high above the earth's surface. As charged particles from the sun's "solar wind" spiral through the earth's magnetic field toward its poles, they collide with one another and with atoms in the earth's upper atmosphere. The energy of such collisions can excite the atoms involved and cause them to emit light.

348. What color is the sun as viewed from outside our atmosphere?
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The sun appears bluer when viewed from outside our atmosphere. The earth's atmosphere scatters a substantial fraction of the violet and ultraviolet light in sunlight, leaving a reddened sun disk to our view. Without that scattering, the sun's disk will appear to contain more blue and ultraviolet light.

349. What is black light and how does it work?
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Black light is ultraviolet light. You cannot see it so a room illuminated only by ultraviolet light appears dark or "black". However any fluorescent materials in the room (e.g. brighteners in your clothes) will absorb the ultraviolet light and reemit it as visible light. That is why things with fluorescent pigments on them glow when illuminate by black light.

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