Hot answers tagged

44

Your approach is incorrect. You cannot do this calculation by considering that the energy absorbed by the object is converted into a change in gravitational potential energy. For one thing the object would just get hot and radiate away most of the energy and for another this is a dynamical problem, you have to be able to accelerate the object upwards. What ...


8

Your consideration implies you have a device that can convert laser light with 100% efficiency and convert it to mechanical power. This is theoretically possible, but there is not much of physics. You still need some ladder to climb along, and building a space elevator requires much tougher materials than we currently know. In practice, you would have to ...


8

Lookup Optical Tweezers. The limitations of the technique are due to the damage thresholds; see laser ablation. This idea has been used extensively in science fiction, especially when implemented as solar sails. It's even practical for some applications, as noted in the article. But laser propulsion from the ground suffers losses due to the atmosphere. ...


6

Having read Madan Ivan's (nice) answer and the comments that follow it seems that your confusion stems from your assumption that the many standing modes are already in phase with one another. You said in a comment above that "only by boundary conditions there will have a place where sin are in phase", but the boundary conditions only dictate which modes can ...


6

The temperature limit for laser cooling is not related to gravity but to the always-present momentum kick during absoprtion/emission of photons. Ultracold atom experiments typically use laser cooling at an initial stage and afterwards evaporative cooling is used to reach the lowest temperatures. In evaporative cooling the most energetic atoms are discarded ...


4

An inverted laser medium can be used to amplify light in general so in principle this is perfectly possible. The output would however inherit the properties of the sun light and you would not get a laser like type of radiation. What you also have to keep in mind is the limited gain bandwidth of typical laser materials. Ti:sapphire is an example of a very ...


4

Laser is stimulated emission of highly energetic photons. Fundamental use of laser is heating, propulsion is very distant aim which lasers can achieve. Few kW rating lasers can actually lift the mass (very small values though) because incident energy beam has momentum associated with it. Your assumption is not correct as you are comparing heat energy with ...


4

In response to the questions which you included in a comment above. There were lots of questions there, and it's usually best to try and formulate them into a single, more concise question, or to ask them separately. I would have included this as a comment rather than an answer, but it is longer than the character limit. 1) Whilst I don't usually like to ...


3

I have a lot of experience with CVD and sputtering, but limited experience in PLD; however, several of my colleagues did this all the time in our shared a laser lab. When they were attempting to reproduce a specific result all of the parameters had to be systematically varied, from the laser fluence to the substrate conditioning and temperature, and more. ...


3

Sun pumped laser The two most studied lasing media for solar-pumped lasers have been iodine,1 with a laser wavelength of 1.31 micrometers, and NdCrYAG, which lases at 1.06 micrometers wavelength... The largest solar-pumped laser is currently being operated by a research facility in Uzbekistan. It is a 1 MW solar input power NdYAG type laser, ...


2

Total length. The wavelength is already provided as 632.8nm.


2

I have no experience with either CVD or PLD, but it was interesting to think about this question. In a humble attempt to build on Peter Diehr's answer, here is some theory (at a very heuristic and simplified level). The deposition of each new layer can be thought of as being governed at large scales by some mixture of Laplacian and Eden growth in two ...


2

There are loads of things you can do with lasers. However, maybe this one might be of interest? It's called "Laser Audio Interferometer" I tried this and it's simple. The mirror reflects the laser beam back down the bore of the HeNe laser tube, forming a second optical cavity external to the laser. Or use a laser pointer instead; I've been told ...


2

First, there is more ways to mode-lock the laser than just the Q-switch. Second -your second paragraph is not correct. If you would just have laser without mode-locking parts, one mode would suck in all the energy from the active media. This is described in pretty much every book on how lasers work. In one sentence this is because amplification in active ...


2

The first thing you should do is cover your detector with an optical filter that only allows through light with the same wavelength as the laser you are looking for.


2

If cosmological expansion applies on the scale of the earth moon system, then in some short period of time $\delta t$ the distance between the earth and moon increases from $r$ to $r+\delta r$. So the force of gravity between the bodies changes to: $F+\delta F=\frac{GMm}{(r+\delta r)^2}\approx\frac{GMm}{r^2}\left(1+\frac{\delta ...


1

Regarding your 1st question, the two options you give appear to be the same. Using the wave picture, the emitted wave is in phase with the incident wave, and the two waves (which have the same frequency and are in phase) interfere constructively to make a wave of larger amplitude. The emitted wave is in phase with the incident wave because the excited atom ...


1

At it's simplest level, satellite tracking is a 3D version of triangulation. Suppose one were able to have 3 observers at the vertices of a triangle on the ground all simultaneously and instantaneously measure the distance from themselves to the satellite. Then one could use the known coordinates of the 3 observers along with the measurements they each ...


1

Focal length of Lens 2 is $d_5$, because parallel rays converge at a distance of d5 from lens 2.Focal length of lens 1 is $d_1$ beacuse diverging rays become parallel to optical axis at a distance of d1. Focal length of system is $d_2+d_3+d_4+d_5$ FROM LENS 1.


1

Note that the paper does not really claim that the Heisenberg-Langevin formalism is more powerful. All it says is that the analysis (emphasis mine), based on the master-equation approach ... precludes consideration of effects associated with atomic dynamics So, it is not the tool which is lacking, but how it has been used. In section V, the author ...


1

Ultrashort pulses impinging a semiconductor usually generate some level of THz radiation. Electron-hole pairs in a free standing semiconductor can be generated by resonant absorption or by two-photon absorption, depending on the wavelength. They create a transient radiating dipole through the photo-Dember effect. If lacking central symmetry, the ...


1

Semiconductor light emitters are made of such materials, which have quite large index of refraction. This makes it hard for light to exit the emitter — due to Fresnel equations and low index of refraction of air. In a laser the light mostly goes back and forth between two mirrors, and reflections only help the lasing. So the light either exits from a tiny ...


1

In principle there is nothing wrong with your idea. But I think it is not viable by DIY, at least not with the results I guess you'd be expecting, with such a small understanding of the subject (read: Wikipedia is definitely not enough, and if you were to do things seriously the Sun would be the last source you'd be thinking of). First of all, any lasing ...


1

Cover the 'target area' with a frosted sheet as the comments suggest; I would recommend placing the sensor inside a cylindrical, or better, spherical "can" and covering the inside of the can with similar reflective diffusing material. Essentially what I'm describing is an "integrating sphere," which costs more than you want to know :-( . The idea is that ...


1

You're conflating two different views on the description of the attosecond pulse train; in particular, you're flitting back and forth between the time-domain and frequency-domain descriptions, and it's not doing you many favours. Let's look at a few things first: The total field is $$ E(t) = \cdots + ...



Only top voted, non community-wiki answers of a minimum length are eligible