# Tag Info

57

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 ...

40

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 ...

25

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 ...

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: $$1\:\mathrm{ångström} = 1\:\mathrm{Å} = 10^{−10}\:\mathrm{m} = 0.1\:\mathrm{nm} = 100\:\mathrm{pm}.$$

16

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

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...

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 $$... 15 As FrankH said, it's actually energy that determines color. The reason, in summary, is that color is a psychological phenomenon that the brain constructs based on the signals it receives from cone cells on the eye's retina. Those signals, in turn, are generated when photons interact with proteins called photopsins. The proteins have different energy levels ... 14 I've found some sources. Mathematical To start with, as for the mathematical notion of "beats", it seems that one Ibn Yunus (c. 950-1009) was responsible for first demonstrating the trigonometric identity$$ \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 ... 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 ... 13 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 ... 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 ... 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 ... 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 (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 ... 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) $$To calculate average energy transport we note that the average value of sin^2(anything) ... 8 AM radio typically transmits at around 1 MHz, FM radio at about 90 MHz. Measurements of the RF spectrum of lightning strikes show a falloff with frequency of about 20 dB per decade in that frequency range, so with FM about 2 decades above AM, you'd expect AM to have about 40dB higher interference from a lightning strike. In addition to that, FM signals ... 8 A. All light sources (even lasers) are subject to a diffraction limit, so any light beam will eventually diverge with an angle \theta given by$$\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

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 ...

8

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 ...

8

The electromagnetic spectrum does range between (almost) zero and (almost) infinity. It's just that your eyes are sensitive to a very small part of it (from about 380 nm to about 800 nm). At the lowest frequencies, it becomes difficult to recognize the signal from background fluctuations. From this site: "Gamma-rays are detected by observing the effects ...

7

It can be done, but there's some trade offs. Larger speakers are better at moving longer wavelength (low frequency) waves. When you try to combine a bunch of small surfaces in different locations to recreate a single wave you end up with a some random interference where the wave is stronger or weaker (in 3d-space) (see phased-array antenna for some ...

7

For the first question, yes. Because the surface of the sun is close to a blackbody emitter, it radiates at all wavelengths below the peak. So radio waves are included. However, the longer the wavelength, the less the power that is put into that portion of the spectrum. Radio is so far from visible light on the EM spectrum that the solar radiation in ...

7

The way it works has nothing to do with your body. Remotes have their antenna as a more or less circular trace on the board (a loop antenna). The strongest signal is when the top or base of the remote is pointed at the receiver. The weakest signal is when the fob is pointed 90 degrees away, such as when pointing it like a TV remote. Guess which way most ...

7

The physics is actually much easier than it seems at first glance. Power generators are engines just like the everyday ones we see all around in our cars, lawnmowers, snowblowers, etc. Except for new power sources like some wind and solar systems with electronic inverters, the vast majority of power is supplied by large rotating AC generators turning in ...

7

When we think of light, we can describe it as an electromagnetic wave or as a flux of particles - photons. The latter description is more fundamental: If you could have a light source with sensitive enough intensity knob, then after just turning it on (minimum intensity), you'd be sending out photons one by one. I believe that answers to your deep questions ...

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