# Tag Info

58

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 ...

41

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 ...

28

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 teh wavelength changes according ...

24

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", ...

23

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 ...

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

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, ...

16

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.

15

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 $$... 15 As FrankH said, it's actually energy that determines color. The reason, in summary, is that color is a psychological phenomenon that the brain constructs based on the signals it receives from cone cells on the eye's retina. Those signals, in turn, are generated when photons interact with proteins called photopsins. The proteins have different energy levels ... 15 For almost all detectors, it is actually the energy of the photon that is the attribute that is detected and the energy is not changed by a refractive medium. So the "color" is unchanged by the medium... 14 Have a look into the Nyquist theorem. The sampling frequency needs to be at least double the rate of the sampled frequency. I.e. that's why the human ear can hear up to ca. 20kHz and the CD samples at 44.1kHz. Wikipedia Nyquist-Shannon Theorem What do we hear instead if we do listen to (originally) 5 Hz to 20 kHz music through the phone? Is everything ... 14 I've found some sources. Mathematical To start with, as for the mathematical notion of "beats", it seems that one Ibn Yunus (c. 950-1009) was responsible for first demonstrating the trigonometric identity$$ \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 ... 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 ... 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 ... 13 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 ... 12 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. 11 An ideal resistor is defined as the two-terminal circuit element where the voltage across is proportional to the current through: V_R = R \cdot I_R and the constant of proportionality, R, is, well, constant. A physical resistor has at least series inductance and parallel capacitance and can be modelled with ideal circuit elements as follows (for ... 11 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 ... 11 So, we need data of from ears. An audible sound has an minimum intensity of I_0\approx10^{-12}W/m^2. This shows how sensitive our ears really are. A way to see it is to use that intensity to calculate the total variation of air displacement. If you do that, we will have about \Delta u\approx1.1\cdot 10^{−11}m. This is 0.11 angstroms! This is smaller ... 10 A human eye may only distinguish thousands or millions of colors – obviously, one can't give a precise figure because colors that are too close may be mistakenly identified, or the same colors may be mistakenly said to be different, and so on. The RGB colors of the generic modern PC monitors written by 24 bits, like #003322, distinguish 2^{24}\sim ... 10 The colors which are perceived by people are defined by the degree to which the light will excite the red,green, and blue photoreceptors in the cone cells of the eye. There are only three discrete colors we can perceive, and they are red, green, and blue. The statistics of the relative and absolute excitations, the amount of red,green, and blue averaged over ... 10 (This is an intuitive explanation on my part, it may or may not be correct) Symbols used: \lambda is wavelength, \nu is frequency, c,v are speeds of light in vacuum and in the medium. Alright. First, we can look at just frequency and determine if frequency should change on passing through a medium. Frequency can't change Now, let's take a ... 10 Say if I transmit: \sin(2\pi x) And separately: \sin(2\pi x\times 2) Does it end up as a single wave of: \sin(2\pi x)+\sin(2\pi x\times 2)? Yes, that's exactly how it works. This is called superposition. There are electromagnetic waves at hundreds of different frequencies all filling the air simultaneously. The way something like a ... 9 A. All light sources (even lasers) are subject to a diffraction limit, so any light beam will eventually diverge with an angle \theta given by$$\theta \approx \frac{\lambda}{A_T}$$where \lambda is the wavelength of the light and A_T is the aperture of the light beam source (and "eventually" means for distances much greater than A_T). Any beam ... 8 Your voice, like any sound, is a combination of many frequencies. Physically, your voice consists of pressure waves. If we plot the pressure as a function of time, we see that it goes up and down in a way that looks somewhat random. You can measure these pressure waves with a microphone, then visualize them with an oscilloscope. Here's a Youtube video ... 8 AM radio typically transmits at around 1 MHz, FM radio at about 90 MHz. Measurements of the RF spectrum of lightning strikes show a falloff with frequency of about 20 dB per decade in that frequency range, so with FM about 2 decades above AM, you'd expect AM to have about 40dB higher interference from a lightning strike. In addition to that, FM signals ... 8 String theory assumes that lorentz covariance is a perfect symmetry of our world. If that is true, it means a single photon is allowed to have an arbitrary energy, even greater than Planck length. You need at least two photons that are not parallel to have a rest frame where something like a Planckian black hole might be generated that will absorb them. But ... 8 All you need is quantum mechanics, i.e. that nature in the microcosm is dual,sometimes it can manifest wave properties and sometimes particle properties. It depends on the measurement/experiment if the wave or the particle nature will manifest itself. Electrons manifest this duality: in the two slit experiment their wave nature appears governed by the de ... 8 If you have some electromagnetic wave e.g. a plane ave:$$ E = E_m sin(kx - \omega t) $$then the energy transport is given by the Poynting vector. For the plane wave above the energy transport works out to be:$$ S = \frac{1}{c\mu_0} E_m^2 sin^2(kx - \omega t)  To calculate average energy transport we note that the average value of sin$^2$(anything) ...

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