# Tag Info

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The photons move at the speed of light in a straight line from the laser to the moon and back. The spot on the moon can move faster than light. There is no law against that. The spot is not a physical object, just an image. When you turn your wrist nothing happens to the photons which are already on the way to the moon - they continue on the same trajectory. ...

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The ball is probably glowing because it has strontium aluminate in, which produces light by phosphoresence. It's a characteristic of phosphorescence that the light emission is quite long lived. This happens because when you shine light onto a phosphor the light promotes it into an excited state that subsequently decays by interactions with the solid lattice ...

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There are two thermodynamic aspects to laser cooling that are worth mentioning. The first, as others have noted, has to do with the frequency of the light that is absorbed and emitted. In Doppler cooling, the laser is tuned slightly below the frequency that the laser wants to absorb. An atom moving toward the laser sees that light shifted slightly up in ...

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Dear Thomas, the diameter of the beams of these HeNe lasers is between 0.5 and 1 millimeter, so the power 1 mW is coming to $10^{-6}$ squared meters or so. The ratio of power and area is $10^{-3}/10^{-6} = 10^{+3}$ Watts per squared meter. On the other hand, when a 3W LED is watched from the distance 0.1 meters, the power of 3 W is divided to $4\pi R^2 = ... 11 This paper seems relevant to your question. If I'm reading the abstract correctly, the answers to your questions are: Q: It seems that if the coherence length of a laser is big enough, it is possible to observe a (moving) interference picture by combining them. Is it true? A: Yes Q: How fast should photo-detectors be for observing of the ... 11 There is a limit to how small you can focus an ideal single-mode laser beam. The product of the divergence half-angle$\Theta$and the radius$w_0$of the beam at its waist (narrowest point) is constant for any given beam. (This quantity is called the beam parameter product, and is related to the$M^2$beam quality measure you may have heard of.) For an ... 9 Not all lasers are inefficient. Infrared diodes are quite efficient. This paper shows 64% electrical to optical conversion efficiency: http://www.jdsu.com/ProductLiterature/paper_hipower_910_980_laser_diodes.pdf An argon gas laser on the other hand is quite inefficient. 8 Yes. 100W CO2 laser is doable at home, and some in fact did that. 100W one will BURN really well. No other types of power lasers are doable at home. (well, probably there is also killing 200 DVD-RW drives and collimating them all - I am actually doing that, I have 45 RW drives ;-) ) The only problems is that you still need few rare things like IR mirrors ... 8 Some laser rangefinding uses a retroreflector, which will bounce the laser light back in the direction it came regardless of orientation. Otherwise, lasers operate at a very specific frequency, so the signal/noise ratio only needs to be strong enough to be detectable at that frequency. If you shine a normal laser pointer on a wall, even if the wall is ... 8 It is possible to do diffractive optics really cheap - either with photography (with a non-digital camera) or a widely available DTP polygraphic technique. What you need is to: Make a Fourier transform of an image you want to have (it should be relatively simple). Make an image which is white when the real part of the FT is positive, and black - when it is ... 8 Human color vision is based on four types of receptors in the retina: rods, and three types of cones. Their response to different wavelengths is shown in this graph: . It shows clearly how certain wavelenghts, mostly around the yellow-green portion of the spectrum, are absorbed more strongly, and by more types of cells, than the rest. So it is normal ... 7 The most widespread type of laser cooling is "Doppler cooling". The laser light, coming from the sides, is adjusted to a frequency that is slightly below the natural frequency of the atom. So the atom will only absorb the light if it is moving against the laser beam. Consequently, the velocity will drop. When it absorbs the photon, the energy goes to its ... 7 As far as I know you have to worry about how much energy being deposited per surface area. And the area of the "hotspot" of the laser can be very small. So the deposited energy it enough to kill cells on your retina. Actually there is a whole article on Wikipedia about it. 7 Detectors as fast as 50 GHz can be easily bought (if you have the money :P). This means that if the difference in the frequency of the lasers is smaller then 50 GHz or wavelength difference is smaller than 60 pm then you can detect the beating using these fast detectors. This wavelength difference can be achieved (sorry but I am not in the mood of finding a ... 7 Typically, a laser will damage an optical surface in one of two ways. The first is just what you would expect: the laser heats the material up until something bad happens. The second is also pretty simple, but less common because (AFAIK) it is really only a problem with very short pulses (on the order of femtoseconds). In this case a small but rapidly ... 7 so only a tiny fraction of beam energy is reflected back to the device. This tiny fraction is enough. With respect to ambient light: One can modulate the laser beam, and filter the the voltage of the receiving photodiode for this modulation frequency and phase. Another precaution is to have a light filter in front of the receiving photodiode which ... 6 The usual way linear polarisation is measured is by shining polarised light onto a polarising filter, rotating that filter and then using Malus' law to fit the data to a$I_0 cos^2(\theta_{beam} - \theta_{polariser})$shape. By finding the angular position of the intensity peak we can infer the angle of polarisation of the incoming beam. Now, assume we ... 6 can You read German? here http://de.wikipedia.org/wiki/Laserschneiden all Your questions are answered. The English pendant is much shorter and does not deal Your problems. In case You can't read that, some short excerpt: Main problem with copper and Aluminium are high reflectivity at 10 µm, the high thermal conductivity and and no "assistance" from ... 6 It depends on how big a pencil you're thinking about. There's no fundamental reason why radio waves can't be collimated in the same sort of way that visible light beams are. In fact, some radar systems send out fairly collimated beams at radio frequencies. If you want to make a radio-wave beam that is the same size as a typical laser beam, though, you're ... 6 Laser light is spatially and temporally coherent. The stimulated emission is mainly responsible for the temporal coherence. So the answer is yes, you can create an electromagnetic beam that is spatially but not temporally coherent by placing a pinhole close to the source, and then another pinhole in the far field of the first pinhole. This beam will not ... 6 The main problem about a rigorous solution to such a scattering proplem is that computations are extremely demanding. Just imagine you have a wavelength$\lambda$of some$400$nm to$700$nm for visible light (from here): Now, to do physically meaningful simulations, you will need a sub-wavelength lattice which makes any computational cell above, say ... 6 Actually V. A. Fabricant was given credit for his laser idea by both Dr. Charles Townes, scientist/inventor of the first working maser and by Dr. Ted Maiman, scientist/inventor of the first working laser. I happen to be writing a book about Maiman, hence my interest in addressing myself to this question. On Pg 61 of Townes autobiography, "How the Laser ... 6 The word "stimulated" means that the emission of the photon is "encouraged" by the existence of photons in the same state as the state where the new photon may be added. The "same state" is one that has the same frequency, the same polarization, and the same direction of motion. Such a one-photon state may be described by the wave vector and the polarization ... 5 The Coherent Chameleon titanium-sapphire laser tunes over a pretty enormous range, mostly in the IR (680 to 1080 nm). With a frequency doubling crystal, it covers most of the visible (340 to 540 nm), and if you add an OPO, it can tune over the entire visible range with frequency doubling. Supercontinuum sources cover the entire visible spectrum ... 5 The traditional method is with a photomultiplier tube or PMT. These are very expensive but are easily recognized and you might try hanging around the auctions at the local research university to see what comes up. More modern is the Avalanche photodiode or APD. Amazingly, my favorite electronics supplier, Digikey, has them, at this time as cheap as$942. ...

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Interestingly exicmer laser development was originally funded, in part, due to interest in inertial confinement fusion. Original excimer lasers required: rare-gas atoms (e.g., Xe) at high pressures, 10 atm; very high voltages, 400 keV, were required to create an electron beam through a metal foil; high current densities, on the order of 1000 amps/cm$^2$, ...

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For questions like this I always recommend finding the relevant article on Sam's Laser FAQ, which is an incredible practical resource. However,The short answer is that they are much better than standard diode lasers on all three points. The emitted beam from an Nd:YAG or Nd:YVO crystal is very pure 1064 nm light and the nonlinear crystal, typically ...

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Thanks to @Manishearth for the edit In normal phosphorescence, the atoms are in a "metastable" state--where electrons are in a higher energy level, but do not immediately come to ground state due to partial stability. The electrons come down slowly, giving rise to the (relatively) long lasting glow. The IR light frees away the electrons from the shallow ...

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Yeah, Neils Bohr and John Von Neumann were skeptics: Many prominent physicists thought it could not even work, based on their knowledge of physical principles. In quantum mechanics, the uncertainty principle developed by Einstein, says that the energy (and therefore the frequency, by E=hv) of a photon can't be known to great precision in a short time. In ...

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Cutting glass with a water jet uses grains of hard material (such as sand but occasioanlly harder minerals or diamond) to grind away at the glass one particle at a time. The water jet is just used to carry the sand to the glass at high speed and remove it and the eroded glass. As the poster says, it also cools the glass and so prevent cracking. Cutting ...

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