I don't know much about light-emitting diodes, but I imaging if you had a panel of RGB diodes you could produce any wavelength of color within the visible light spectrum. However, if I also wanted to generate specific wavelengths of UVA or UVB (anywhere from 290 to 400nm), could I also accomplish this using diodes? Essentially, I am interested in making a small panel of diodes in which I could produce any specific wavelength of visible light, UVA, or UVB.

Thanks in advance.

What is the significance about the bell shape, when its hit at the rim it rings/produces sound better than other shaped objects? If so could anyone explain a little bit on it.

EDIT: From the suggestions in the comments, clarification for the term "sound better": Sound more effective for the purpose which bells are created for. (Thanks Justin)

I know that string theory is still under heavy development, and as such it still can't make predictions (or not that many predictions anyways).

On the other hand, it is clear from the number of years this theory has been under development and from the large number of theoretical physicists studying it, that it is considered a good and viable candidate as a quantum gravity theory.

So, what is the evidence that this is true? Why is it considered such a good candidate as the correct quantum gravity theory?

Without wanting to sound inflammatory in the least, it has been under heavy development for a very long time and it's still not able to make predictions, for example, or still makes outlandish statements (like extra dimensions) that would require a high amount of experimental evidence to be accepted. So - if so many people believe it is the way to go, there have to be good reasons, right? What are they?

I read an article which tells power consumption by many devices. It say that a desktop computer (computer and monitor) use 400 to 600 watt.
While when i checked my computer and monitor with meter, it was about 60 + 60 = 120 watt (computer + 17" CRT monitor) after loading windows xp and running an application. The voltage is 220V here.
Which one is correct? How much power does it consume?

Q9

Provided the amplitude is sufficiently great, the human ear can respond to longitudinal waves over a range of frequencies from about 20.0 Hz to about 20.0 kHz. (a) If you were to mark the beginning of each complete wave pattern with a red dot for the long-wavelength sound and a blue dot for the short-wavelength sound, how far apart would the red dots be? m How far apart would the blue dots be? cm (b) In reality would adjacent dots in each set be far enough apart for you to easily measure their separation with a meterstick? Yes No (c) Suppose you repeated part (a) in water, where sound travels at 1480 m/s. How far apart would the red dots be? m How far apart would the blue dots be? cm Could you readily measure their separation with a meterstick? Yes No

What are the different death scenarios for a black hole? I know they can evaporate through Hawking radiation - but is there any other way? What if you just kept shoveling more and more mass and energy into the black hole?

Is there any reason to believe that any measure of loudness (e.g. sound pressure) might have an upper boundary, similar to upper limit (c) of the speed of mass?

A bicyclist makes a trip that consists of three parts, each in the same direction (due north) along a straight road. During the first part, she rides for 27.8 minutes at an average speed of 6.13 m/s. During the second part, she rides for 44.7 minutes at an average speed of 3.16 m/s. Finally, during the third part, she rides for 8.19 minutes at an average speed of 19.7 m/s. (a) How far has the bicyclist traveled during the entire trip? (b) What is the average speed of the bicyclist for the trip?

It's a Christmas time and so I hope I'll be pardoned for asking a question which probably doesn't make much sense :-)

In standard undergraduate nuclear physics course one learns about models such as Liquid Drop model and Shell model that explain some properties of nucleus.

But as I understand it these models are purely empirical and most importantly incompatible. Perhaps it's just my weak classical mind but I can't imagine same object being described as a liquid with nucleons being the constituent particles floating freely all around the nucleus and on the other hand a shell model where nucleons occupy discrete energy levels and are separated from each other.

Now I wonder whether these empirical models are really all we've got or whether there are some more precise models. I guess one can't really compute the shape of the nucleus from the first principles as one can do with hydrogen atom in QM. Especially since first principles here probably means starting with QCD (or at least nucleons exchanging pions, but that is still QFT). But I hope there has been at least some progress since the old empirical models. So we come to my questions:

Do we have a better model for description of a nucleus than the ones mentioned?

How would some nuclei (both small and large) qualitatively look in such a better model? Look here means that whether enough is known so that I could imagine nucleus in the same way as I can imagine an atom (i.e. hard nucleus and electrons orbiting around it on various orbitals).

What is the current state of first-principles QCD computations of the nucleus?

Theory predicts that uniform acceleration leads to experiencing thermal radiation (so called Fulling Davies Unruh radiation), associated with the appearance of an event horizon. For non uniform but unidirectional acceleration the shape of the experienced radiation changes from thermal to other spectral densities, but also is predicted to exist. But suppose the acceleration is periodic and oscillatory, i.e. no permanent horizon persists? In particular, what about the case of harmonic motion, for a full cycle, half a cycle, etc.?

Here is an even simpler related problem that makes the apparent paradox easier to see. Suppose at proper time t=0, I accelerate at constant acceleration k in the x direction for t0 seconds, presumably experiencing Unruh radiation. Then I accelerate with acceleration -k, (in the -x direction,) for 2*t0 seconds, seeing more Unruh radiation coming from the opposite direction, and then I finish with with acceleration +k for the final t0 seconds. At the end of the 4*t0 proper seconds, I'm back where I started, at rest, without any event horizon. Was the Unruh radiation I felt when reversing acceleration secretly correlated or entangled with the radiation I initially and finally saw? Otherwise, from a more macro scale, I didn't actually necessarily move much, and the acceleration event horizon was instantaneous, evanescent and fleeting, so whence arose the Unruh radiation?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them.

How and why is this possible?

Also, why can't anything travel faster than light?

A wheel of radius b is rolling along a muddy road with a speed v. Particles of mud attached to the wheel are being continuously thrown off from all points of the wheel. If v2 > 2bg, where g is the acceleration of gravity, find the maximum height above the road attained by the mud, H = H(b,v,g).

I heard somewhere that quarks have a property called 'colour' - what does this mean?

I am trying to get a common understanding from these two previous questions:

Why does the mass of an object increase when its speed approaches that of light?
What happens if light/particles exceeded the speed of light for a particular medium (sic)

Does the increase of mass occur only if the particle approaches c (speed of light in a vacuum) or if it simply approaches the speed of light in its current medium? For example, does the mass of charged particles increase during Cherenkov radiation?

The nose of an ultralight plane is pointed south, and its airspeed indicator shows 39m/s . The plane is in a 12m/s wind blowing toward the southwest relative to the earth.

Question A:

Letting x be east and y be north, find the components of v? P/E (the velocity of the plane relative to the earth).

Question B:

Find the magnitude of v? P/E.

Question C:

Find the direction of v? P/E.

Any help is appreciated!