# Tag Info

128

Why is mains frequency 50Hz and not 500 or 5? Engine efficiency, rotational stress, flicker, the skin effect, and the limitations of 19th century material engineering. 50Hz corresponds to 3000 RPM. That range is a convenient, efficient speed for the steam turbine engines which power most generators and thus avoids a lot of extra gearing. 3000 RPM is also ...

67

This is a really interesting question. It turns out that your body is reasonably conductive (think salt water, more on that in the answer to this question), and that it can couple to RF sources capacitively. Referring to the Wikipedia article on keyless entry systems; they typically operate at an RF frequency of $315\text{ MHz}$, the wavelength of which is ...

50

I'm not going to address the production mechanism,1 just the nature of the "sound" in this case. What you think of as the hard vacuum of outer space could just as well be seen as a very, very, very diffuse, somewhat ionized gas. That gas can support sound waves as long as the wavelength is considerably longer than the mean free path of the atoms on the ...

43

The limitation you're hearing has been part of the phone network since long before digital sampling had any part in the telephone system. It is related to the fact that the connection from a land-line phone in your house or office back to the "central office" of the phone company is essentially a continuous connection through a pair of wires. There's ...

42

Do low frequencies carry farther than high frequencies? Yes. The reason has to do with what's stopping the sound. If it weren't for attenuation (absorption) sound would follow an inverse square law. Remember, sound is a pressure wave vibration of molecules. Whenever you give molecules a "push" you're going to lose some energy to heat. Because of this, ...

35

Colour is defined by the eye, and only indirectly from physical properties like wavelength and frequency. Since this interaction happens in a medium of fixed index of refraction (the vitreous humour of your eye), the frequency/wavelength relation inside your eye is fixed. Outside your eye, the frequency stays constant, and the wavelength changes according ...

35

As the other guys have already covered most of the topic, I'd like to quote some things. Light can't escape only from the inside of event horizon because it has already fallen into it. But after reading the article now, we could indicate some points. The article specifically says a "supermassive blackhole". They're a way too bulk in size when compared to ...

32

The definition for the cesium clock is: 9192631770 cycles per second is frequency of the radio waves which cause maximum resonance, a physically measurable condition, in the cesium atoms. This corresponds to a particular tuning of the radio. Keeping it tuned provides the reference frequency cited.

28

The electric and magnetic fields have to remain continuous at the refractive index boundary. If the frequency changed, the light at each side of the boundary would be continuously changing it's relative phase and there would be no way to match the fields.

28

You're correct and the video is mistaken. In fact, if cesium atoms were constantly oscillating between the two hyperfine states, cesium beam clocks wouldn't work at all! In its simplest form, a cesium beam clock uses a magnet to separate a stream of atoms into two streams based on their hyperfine state; one state is selected to continue down the tube to be ...

27

We can consider four aspects of your question: Why do most events generate sound? What sounds get propagated? What does it take for sound to be detected? Has evolution got anything to do with this? 1 - generating sound Most of the sounds you describe are "broad band". Remember that a delta pulse (short sharp shock) is basically "all frequencies", ...

26

In the end, the choice of a single specific number comes from the necessity to standardize. However, we can make some physical observations to understand why that final choice had to fall in a certain range. Frequency Why a standard? First of all, why do we even need a standard? Can't individual appliances convert the incoming electricity to whatever ...

24

According to Wikipedia the frequency range of the plain old telephone service is 300Hz to 3.4kHz. So any music you listen to will be missing the low frequencies and missing the high frequencies. If you remember back to the last time you heard hold music on the phone you'll probably remember that it sounded a bit muffled, but I have to say that it's still ...

23

For almost all detectors, it is actually the energy of the photon that is the attribute that is detected and the energy is not changed by a refractive medium. So the "color" is unchanged by the medium...

22

You don't. You actually hear the high frequency notes from headphones. The bass really doesn't travel at all well, but the attack noise from the drum or bass guitar is what leaks from headphones. This is why on the tube you hear "tsss tsss tsss tsss" and very little else. From @leftaroundabout's answer on the post that valerio92 linked: Normal ...

21

As promised in the comments to my answer, I went out and measured the effect in a number of different configurations (a couple of days later than promised :-)). For those of you who just want the conclusions, here they are: The remote seems to work better when held to the head though the improvement isn't as marked as one might have expected from a google ...

21

It is an ångström, a unit of length commonly used in chemistry to measure things like atomic radii and bond lengths. Although not an official SI unit, it has a simple relationship to the metric units of length: $$1\:\mathrm{ångström} = 1\:\mathrm{Å} = 10^{−10}\:\mathrm{m} = 0.1\:\mathrm{nm} = 100\:\mathrm{pm}.$$

21

The human voice box produces a fundamental frequency and its harmonics because the mechanism is like that of a relaxation oscillator. However, we have limited control over the relative amplitude of the harmonics (we do have some - that is how we change the "color" of a tone we sing, and the sound of vowels). In order to produce the Shepard scale, you need ...

20

As FrankH said, it's actually energy that determines color. The reason, in summary, is that color is a psychological phenomenon that the brain constructs based on the signals it receives from cone cells on the eye's retina. Those signals, in turn, are generated when photons interact with proteins called photopsins. The proteins have different energy levels ...

19

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 ...

18

Have a look at an announcement from LIGO where they describe the experiment. The first plot shows the frequencies detected. The original waves are redshifted. Estimated source parameters for GW150914. We report the median value as well as the range of the 90% credible interval. Masses are measured in the source frame; to convert masses to detector ...

17

Lorentz came with a nice model for light matter interaction that describes dispersion quite effectively. If we assume that an electron oscillates around some equilibrium position and is driven by an external electric field $\mathbf{E}$ (i.e., light), its movement can be described by the equation $$m\frac{\mathrm{d}^2\mathbf{x}}{\mathrm{d}t^2}+m\gamma\frac{\... 16 I've found some sources. Mathematical To start with, as for the mathematical notion of "beats", it seems that one Ibn Yunus (c. 950-1009) was responsible for first demonstrating the trigonometric identity$$ \cos a \cos b = \frac 12 \left( \cos ( a + b) + \cos (a - b) \right )  quoting A History of Mathematics By Carl B. Boyer, Uta C. Merzbach At ...

15

Because the frequency of a sound wave is defined as "the number of waves per second." If you had a sound source emitting, say, 200 waves per second, and your ear (inside a different medium) received only 150 waves per second, the remaining waves 50 waves per second would have to pile up somewhere — presumably, at the interface between the two media. ...

14

There seem to be a lot of human body mechanical models, such as this one: As for applications, I have heard that sub-audio frequency vibrations have been considered as nonlethal weapons for riot control.

14

Have a look into the Nyquist theorem. The sampling frequency needs to be at least double the rate of the sampled frequency. I.e. that's why the human ear can hear up to ca. 20kHz and the CD samples at 44.1kHz. Wikipedia Nyquist-Shannon Theorem What do we hear instead if we do listen to (originally) 5 Hz to 20 kHz music through the phone? Is everything ...

14

The wavelengths that stimulate vitamin D production are between 280nm and 320nm, which is called UVB. You would need to use a detector capable of measuring light in this wavelength. However there is no need, because normal windows are made from soda-lime glass and this transmits no wavelengths shorter than about 350nm. Some Googling will find you the ...

14

A "pure tone" is a sound that has a single sine function as its pressure profile. The human voice is not a pure tone; it is a superposition of many different sine waves with different frequencies and different amplitudes. Here is an image illustrating how many sine waves of different frequencies can combine to make a more complicated waveform like the human ...

13

The speed of light in vacuum is constant and does not depend on characteristics of the wave (e.g. its frequency, polarization, etc). In other words, in vacuum blue and red colored light travel at the same speed c. The propagation of light in a medium involves complex interactions between the wave and the material through which it travels. This makes the ...

13

Do keep in mind that the frequency of light is reference frame dependent. So, for example, the cosmic background microwave radiation would appear as a concentrated gamma radiation source 'in front' to an observer with ultra-relativistic speed relative to the CMB. In other words, light emitted from a body of a particular frequency in that body's frame of ...

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