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43

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. ...


40

How interesting. Presumably the "glow-in-the-dark" effect comes from the decay of a meta-stable excited state. It gets charged by sufficiently energetic photons, and decays slowly because some selection rule prevents a direct transition without an external influence. If this is the case, we can guess that the laser is exciting the meta-stable state to a ...


33

When lasers cut something, they're only cutting in the sense that they're making atoms be not as attracted as they once were to each other. When you get down to the nitty-gritty details, it is not really the same as mechanical cutting. Remember that lasers shoot photons, and when photons hit atoms, they excite electrons. If you excite these electrons ...


31

An alternative way to generate random numbers, that truly is quantum, and also quite easy: put a small radioactive source near a Geiger counter. Radioactive decay is a truly random event in the quantum sense, and is basically not subject to thermal noise at all. For maximum visual impact, replace the Geiger counter with a cloud chamber. That way you can ...


25

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 ...


21

The camera in a smartphone is sensitive to wavelengths of light that the human eye cannot see. Have a look at this picture from this article: This is actually a Canon EOS 40D SLR, but the idea is the same. The human eye can't see any wavelengths longer than about 700nm, but the graph shows the camera can detect light out to 1000nm. So if you project your ...


20

A photon will travel "at the speed of light" until obstructed. From the speed, and elapsed time, you can calculate how far the light will travel. Laser light consists of more than one photon "in phase", which has exactly the same property in this respect, as a solitary photon.


18

Yes easily. Fiber-fed 5-10kW Nd-Yag lasers are commonly used for cutting metal in machine shops. Fibers are so transparent, especially when designed for a single wavelength laser, that the power loss and so heating in the fiber is very small. It's generally less than an optically fed laser where dirt accumulates on the lenses. Many systems have a thin ...


17

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 ...


17

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 ...


14

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 = ...


14

You see it because it travels through air, dust, and a lot of other molecules and particles that can reflect and diffuse it. This, together with focussing, is also the reason for why it cannot travel arbitrary long distances. If you go to vacuum then the laser beam has much less losses, and it can travel much farther as happens in the LIGO interferometers ...


14

If you want to make a small very-hot spot, spatial coherence is important. Contrary to what you say, the sun has quite high spatial coherence (not as high as a laser, but higher than most other bright light sources). That's why you can focus sunlight very well. If you focus sunlight perfectly, you get a spot the shape of the sun. That spot would have the ...


13

Cutting is a process when you deliver energy to break chemical bonds in material that you cut. When you use a saw, you deliver mechanical (kinetic) energy that converts into kinetic energy of particles of the thing you cut, so they can get out of the thing. Laser is just another way to deliver such energy, since the a photon has enough energy to break some ...


13

As previous answers have stated, the wavelength (or frequency) and intensity of the beam are important, as well as the type and amount of impurities in the air. The beam must be of a wavelength that is visible to humans, and fog or dust scatters the light very strongly so that you can see it. However, even in pure, clean air, you will be able to see a laser ...


13

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 ...


12

Theoretically, the photon (or the beam of photons, there really isn't a difference) can go an infinite distance, traveling all the while at a speed $c$. Since photons contain energy, $E=h\nu$, then energy conservation requires the photon to only be destroyed via interaction (e.g., absorption in an atom). There is nothing that could make the photon simply ...


12

It is possible to hit spots in the air with laser pulses from multiple directions in such a way that air molecules in that spot become ionized and emit light, see the technology discussed in this article, along with this demonstration video. And if you just want a 2D screen rather than a 3D display, then of course you can also just use lasers to project it ...


11

If I understood correctly, what you are trying to build is a hardware based random number generator, where you want to use some quantum mechanics-based mechanism to supply the randomness. I'm no experimentalist, thus, take my comments with a grain of salt. Your suggestion is to use Schottky noise from a illuminated photodiode. I believe that it's a pretty ...


11

If you look at reflectances of common materials used to make mirrors with (for example, the topmost graph found on this wiki page), you'll see that not 100% of the light is reflected, especially at the shorter wavelengths. I'm still looking for a somewhat better source for similar curves for household mirrors, but I know that the idea is roughly the same ...


11

I would guess that in this case the word faster means that more data per second can be transferred. This is because light has a much higher frequency than microwave so it can be modulated at a much higher frequency. Microwave frequencies are in the range 1 GHz to 100 GHz while light is around 600 THz, so light can in principle transfer 1,000 to 100,000 times ...


11

Due to Heisenberg uncertainty principle $\Delta x\Delta p\gtrsim \frac{\hbar}2$, one can't really make a quantum have zero momentum in any direction. So you can't say that photons go in the same direction - this is just a simplified description of laser operation. In reality, the thinner the beam, the higher the divergence. Compare e.g. a DPSS laser (e.g. ...


10

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 ...


10

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 ...


10

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 ...


10

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 ...


10

To add to Nathaniel's Answer because (1) it is a good answer and (2) I get nervous recommending radioactive materials handling to anybody I don't know: I would really think about the cloud chamber idea, especially since you're a software guy with a math background. It would need to be inside a darkened container, but you could run a webcam to show what is ...


9

Lasers by definition only emit a single wavelength of light. You use one if you want that wavelength or if you want your photons to be in phase. You don't care about the photon phases, and you want to sample all wavelengths, so a laser is very much the wrong tool. If you just want collimation of the light, mirrors, lenses, or even just well-separated ...


9

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 ...


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.



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