Posted: 10/29/2005 4:17:36 AM EDT
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If you took a flashlight, say a 1 million candlewatt spotlight, and positioned a mirror 1,000 feet away to reflect the light to another mirror right next to the spotlight (so the two mirrors are 1,000 feet away from each other), and then set up, say, 100 more mirrors so the light would theoretically bounce back and forth all the way down the string of mirrors (that are set 1,000 feet apart)...would: A. There be any measurable amount of time between when the spotlight was turned on and when the light showed up on the last mirror in the string of mirrors? Remember, the last mirror in the string might only be 50 feet from the light source (the spotlight), but the distance from the light source to the last mirror (following the reflections off the mirrors themselves) would be 100 X 1,000 feet. B. How much fade (loss) in total light would there be between the light source (1 million candlewatts) and the last mirror in the string? |
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Light travels at something in the neighborhood of 38,000 miles per second IIRC (I could be way wrong on that...but if anything, it is faster) so there would be no noticeable depreciation in speed. Depending on the quality of the mirror, there would be some weakening in the strength of the beam over distance, but with the strength/power of the original beam, it too would be negligible. Then again, I could be completely wrong. 
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186000 miles per second roughly. :P And yes if you had enough mirrors you could see a noticable difference. Thats a lot of damn mirrors though. And with a normal light source there would be a LOT of fade over those kind of distances, even in a vacuum. Thats why these kind of experiments are usually done with lasers. You could take two long mirrors almost exactly parallel to each other, just a slight fraction off, and point a lasrer straight at one of the sides at the 'narrow' end, and the laser would bounce back and forth all the way down to the 'wide' end. If the experiment was done with enough precision in the setup and the mirrors you could probably see a noticable delay without the mirrors needing to be miles and miles apart. But getting back to your orginal question, we have the technology to measure the time between a light turning on and the light getting there in MUCH shorter distances than we are talking about here. It's a whole different thing for it to be perceptible though. |
If we could fabricate metals whose strength depended on the molecular bonds, the world would be a wonderful place. A bazillion rpm. |
The light would become less intense through scattering and diffusion, not because it lost "strength"; there is less light flux between each mirror pair. Now, if you organize that light and keep it coherent between each mirror, you almost built a laser. |
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Even the very BEST mirrors are only about 99 percent efficient. So lose 1 percent or more at every reflection. White light scatters too much anyway. You'll lose more light to scattering than to reflection losses. Try it with a well-collimated laser, and you'll get better results. Light travels about 186,000 miles per second, or 300 million meters per second. Your light path would be 100,000 feet long. The delay would be easily measurable but not noticeable to the human eye. CJ |
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Popular science had an article a few months ago where a guy with an ultra-high speed camera was able to capture images of a light beam between parallel mirrors, before the beam made it to the end. NASA measures the distance (and change in distance) between the earth and the moon by the time it takes a laser beam to bounce off a mirror on the surface of the moon. Obviously not something you could detect with your eye, but electronic equipment can. There is a limit to how quickly your eye can resolve two different occurances. Knowing that amount of time, and the speed of light, you can calculate how far away you need a mirror to reflect light back to you such that you can notice a delay in it's return. |
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Back when I was a young engineering LT, I often pondered how cool it would be to string det cord coast to coast and the 8 minutes it would take to cross (give or take). 30,000 FPS 6 miles per second 3000 miles 500 seconds Just a rough estimate. I think the FPS may be a bit fast. Its been a while. Have to break out the 5-250 again. |
Why are Army engineers always looking for stuff to blow up? I had engineers with me in Iraq when we found a "mine" that was really part of an SA-7. Of course, they wanted to blow it in place anyway. 
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Reminds me of the mirror globe Heinlien wrote about: You are an astronaut in a 20' perfectly vacuum "filled" enclosed globe, with 100% perfect mirror finish all around the inside. Ignore how you got the astronaut in, etc. You turn on a flashlight for 1 second and then turn it off. What happens? |
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Would all the light be intensified and focalized at the exact center of the globe? you would be fried. (I think) |
STOP IT with the "duality of light's behavior" shit!!! Yer gonna gimme a headache!!!! Still, I will take up the "wave" side of this little gem and say that the minute you eliminate the source of the wave then you douse the "candle" so to speak! |
I'm guessing that the light goes away as soon as the light is turned off. |
Now my head hurts. |
I don't think so. If two 100% mirrors are perfectly facing each other and one has a laser behind it that's turned on for one second only, I don't think the laser beam keeps bouncing back and forth even after the laser is turned off. |
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If you're on Earth and you point a flashlight at someone on a planet one light year away. You turn the flashlight on for one second and then turn it off. One light year later that other person will see a one second flash of light from the flashlight. If you turn the flashlight in the reflective sphere one and then off where is the light going to go?? Wouldn't it stay in the sphere being "trapped" as it were?? |
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"Lightwave Testers" (guys that work on fiberoptic equipment, servers, etc, that make up the phone system and internet, which is all the same equipment) have a device that sends laser light down a fiber, and measures the time it takes to return from a break in the line. They get a reading of how far down the line a break has occurred. Then they know where that idiot with the backhoe dug where he was not supposed to dig. Yes, you can measure such things. |
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Yup, you sure can. Personally, I find this to be absolutely amazing, but cops now frequently use "LIDAR" instead of the old radar speed detectors - the new LIDAR can detect your speed very, very accurately (to like within a tenth of a mile per hour) just by measuring how long it takes a beam of light to travel from the emitter/to the car/back to the emitter a couple times over (then measures the amount of time and detects how much closer you are by how much less time it takes for the light to return). It is amazing to me that we have handheld machines that can measure exceptionally accurately how long it takes a beam of light to travel a mere ten or twenty feet. Unbelievable. As for the original mirror question. Light travels 186,000 miles per second, that's 982,080,000 feet per second. Since (as previously stated) you can only see something as short as 1/20th of a second (give or take), you would need at least 1/20th of 982,080,000 feet total distance between all your mirrors. Assuming all your mirrors are placed 1,000 feet apart, you'd need enough mirrors to make the light travel 1,000 feet 49,104 times over to notice even the slightest difference. 50,000 mirrors is a lot of mirrors. |
Not sure what would happen there. I did read somewhere, however, that if you had a mirror-spehere identical to the one you described except really really big with a black hole in the center, and if you shined a light inside the sphere, it would explode. EDIT: as long as we're talking about this, can someone please tell me if light is a wave or a particle? |
It's both, or either. It depends on how you are measuring it. If you measure light as particles, then it's photons. If, on the other hand, you measure it as a wave, it measures as a waveform. Quantum Physics is "Mind numbing" stuff. Tall Shadow 
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First thing, it's candlepower, not candlewatt. Another important thing to know is always be wary of any "candlepower" specs because it is a measure of intensity (lumens/steradian), not light output. Anytime you see a candlepower rating on a flashlight, it's pretty much useless. It's like when you buy air compressors, your little 12V compressor may be able to do 200psi while a large 3 HP may only do 90psi but that doesn't mean the 12V compressor has greater output. For an experiment like this, you'd want to use a laser. To answer your question though... A) Absolutely that's how laser rangefinders work. B) Don't know the numbers off the top of my head but even assuming 100% specular reflection, you got to draw the reduction in solid angles (think light cone) between each mirror and you are dealings with many orders of magnitude of light loss. That doesn't mean you couldn't measure it though. |
In college, I did that with a cable. In the real world, I either just find the break, or determine that there is one and have the cable replaced. 'course, I work with much smaller cables. |
the astronaut's body would absorb the light. If you placed mirrors exactly parallel to each other, the light would bounce back and forth infinately, when you peaked to see if the light was still there, your eye would immediately absorb the light and before your retina could detect the light, it would be gone. no more noticeable light. how's that? |
+1 IIRC in one experiment, they where able to slow light down to  www.msnbc.com/news/242698.asp?cp1=1 |
Astronaut is also in a smaller sphere with a perfect mirror on the outside, along with the flashlight portal. i.e. Nothing to absorb |