# Tag Info

42

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. ...

39

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 ...

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 ...

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 ...

16

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 ...

15

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 ...

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 $4\pi R^2 = ... 14 You see it because it travels through air, dust, and a lot of other molecules and particles that can reflect and diffuse it. This, together with focussing, is also the reason for why it cannot travel arbitrary long distances. If you go to vacuum then the laser beam has much less losses, and it can travel much farther as happens in the LIGO interferometers ... 13 If you want to make a small very-hot spot, spatial coherence is important. Contrary to what you say, the sun has quite high spatial coherence (not as high as a laser, but higher than most other bright light sources). That's why you can focus sunlight very well. If you focus sunlight perfectly, you get a spot the shape of the sun. That spot would have the ... 13 Cutting is a process when you deliver energy to break chemical bonds in material that you cut. When you use a saw, you deliver mechanical (kinetic) energy that converts into kinetic energy of particles of the thing you cut, so they can get out of the thing. Laser is just another way to deliver such energy, since the a photon has enough energy to break some ... 12 There is a limit to how small you can focus an ideal single-mode laser beam. The product of the divergence half-angle$\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$beam quality measure you may have heard of.) For an ... 11 As previous answers have stated, the wavelength (or frequency) and intensity of the beam are important, as well as the type and amount of impurities in the air. The beam must be of a wavelength that is visible to humans, and fog or dust scatters the light very strongly so that you can see it. However, even in pure, clean air, you will be able to see a laser ... 11 I would guess that in this case the word faster means that more data per second can be transferred. This is because light has a much higher frequency than microwave so it can be modulated at a much higher frequency. Microwave frequencies are in the range 1 GHz to 100 GHz while light is around 600 THz, so light can in principle transfer 1,000 to 100,000 times ... 10 Due to Heisenberg uncertainty principle$\Delta x\Delta p\gtrsim \frac{\hbar}2$, one can't really make a quantum have zero momentum in any direction. So you can't say that photons go in the same direction - this is just a simplified description of laser operation. In reality, the thinner the beam, the higher the divergence. Compare e.g. a DPSS laser (e.g. ... 10 Human color vision is based on four types of receptors in the retina: rods, and three types of cones. Their response to different wavelengths is shown in this graph: . It shows clearly how certain wavelenghts, mostly around the yellow-green portion of the spectrum, are absorbed more strongly, and by more types of cells, than the rest. So it is normal ... 10 If you look at reflectances of common materials used to make mirrors with (for example, the topmost graph found on this wiki page), you'll see that not 100% of the light is reflected, especially at the shorter wavelengths. I'm still looking for a somewhat better source for similar curves for household mirrors, but I know that the idea is roughly the same ... 10 Yes. 100W CO2 laser is doable at home, and some in fact did that. 100W one will BURN really well. No other types of power lasers are doable at home. (well, probably there is also killing 200 DVD-RW drives and collimating them all - I am actually doing that, I have 45 RW drives ;-) ) The only problems is that you still need few rare things like IR mirrors ... 9 It is possible to do diffractive optics really cheap - either with photography (with a non-digital camera) or a widely available DTP polygraphic technique. What you need is to: Make a Fourier transform of an image you want to have (it should be relatively simple). Make an image which is white when the real part of the FT is positive, and black - when it is ... 9 The usual way linear polarisation is measured is by shining polarised light onto a polarising filter, rotating that filter and then using Malus' law to fit the data to a$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 ... 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. 9 Light passing through a moving medium undergoes a shift due to the difference in frames between the two media. This problem is quite simple to solve in the frame of the river. In this frame the light is moving at an angle and the river is still. The air is moving relative to the river but since air has an index of refraction of$1$, its movement doesn't have ... 8 Think of the laser process as being similar to melting a substance through a change in state. An analogy might be putting a hot wire on top of an ice cube. It makes a 'slice' by heat, not by cutting. It turns the solid state ice into water and gas which doesn't hold together the same anymore. 8 This is wrong. A wide region (width$\gg\lambda\$) of many, evenly distributed in-phase spherical wave emitters, like the Huygens equivalent of a laser beam's cross section, behaves like a phased antenna array, with the result that destructive interference between the emitters cancels radiation deviating significantly from the beam and reinforces radiation ...

8

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 ...

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

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 ...

8

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.

8

Just to expand on Dr. Motl's answer: On average - when laser cooling is working properly and cooling the atoms - the photons emitted by the atoms are slightly higher energy than the photons absorbed. That's where the energy goes.

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