# Tag Info

49

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 ...

21

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 ...

12

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 ...

11

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 $$... 11 An ideal resistor is defined as the two-terminal circuit element where the voltage across is proportional to the current through: V_R = R \cdot I_R and the constant of proportionality, R, is, well, constant. A physical resistor has at least series inductance and parallel capacitance and can be modelled with ideal circuit elements as follows (for ... 10 A human eye may only distinguish thousands or millions of colors – obviously, one can't give a precise figure because colors that are too close may be mistakenly identified, or the same colors may be mistakenly said to be different, and so on. The RGB colors of the generic modern PC monitors written by 24 bits, like #003322, distinguish 2^{24}\sim ... 9 Say if I transmit: \sin(2\pi x) And separately: \sin(2\pi x\times 2) Does it end up as a single wave of: \sin(2\pi x)+\sin(2\pi x\times 2)? Yes, that's exactly how it works. This is called superposition. There are electromagnetic waves at hundreds of different frequencies all filling the air simultaneously. The way something like a ... 8 A. All light sources (even lasers) are subject to a diffraction limit, so any light beam will eventually diverge with an angle \theta given by$$\theta \approx \frac{\lambda}{A_T}$$where \lambda is the wavelength of the light and A_T is the aperture of the light beam source (and "eventually" means for distances much greater than A_T). Any beam ... 8 AM radio typically transmits at around 1 MHz, FM radio at about 90 MHz. Measurements of the RF spectrum of lightning strikes show a falloff with frequency of about 20 dB per decade in that frequency range, so with FM about 2 decades above AM, you'd expect AM to have about 40dB higher interference from a lightning strike. In addition to that, FM signals ... 8 The speed of light in vacuum is constant and does not depend on characteristics of the wave (e.g. its frequency, polarization, etc). In other words, in vacuum blue and red colored light travel at the same speed c. The propagation of light in a medium involves complex interactions between the wave and the material through which it travels. This makes the ... 8 If you have some electromagnetic wave e.g. a plane ave:$$ 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) ... 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 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 ... 7 (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 ... 7 Your question comes down to whether the EM absorption is a resonant process or not, where resonant means it corresponds to the energy of some excitation of the water molecule. The answer is that it is not a resonant process. Microwave ovens operate at 2.45GHz but the lowest energy transitions of water molecules are rotational transitions, which have energies ... 7 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, ... 6 I'm getting the impression that a good part of this question (and perhaps also this physics.SE question?) arises from a wrong presumption that time and position should be on equal footing in quantum mechanics. They are not. The position \hat{\bf r} is an operator, while time t is a parameter. (Notation: In the following boldface denotes a vector ... 6 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 ... 6 I can't claim any experimental experience in this area (fortunately :-) but I thought it was interesting enough to be worth a bit of Googling. The results suggest there is a difference between shells and bombs. There is an extensive collection of eye witness accounts of WW2 at http://www.bbc.co.uk/history/ww2peopleswar/categories/, and searching this ... 6 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 ... 6 It would depend on damping effects being taken into account or not. Invoking Newton's 2nd Law of motion, a differential equation for the motion of a damped harmonic oscillator can be written (including an external, sinusoidal driving force term): m\frac{d^2x}{dt^2}+2m\xi\omega_0\frac{dx}{dt}+m\omega_0^2x=F_0\sin\left(\omega t\right) Where m is the ... 6 Probably not. A fresnel lens isn't just a rippled surface, it has discontinuities, or straight edges. The area of these edges mostly causes loss of incident power. The optics designer wants a good ratio of its (aspheric) area of use to its unused area at edges. Sound and other vibrations could create sine wave-like ripples on the surface of a liquid, but ... 6 Essentially, the glass breaks because the sound is at the right frequency. Every object has a natural frequency (vibrations per second), at which it prefers to vibrate. This is called the "resonant frequency". If you tap a quality wineglass next to your ear, you'll hear it sing at that frequency. If you stimulate the glass with a sound at that frequency, the ... 5 Is an X-ray an example? An X-ray is not an example of an RF wave. Electromagnetic waves are classified based on their frequencies (or, equivalently, their wavelengths). RF waves are low energy, and have wavelengths from 1 mm to 100 km. X-rays are extremely high energy, and have wavelengths from 0.01 to 10 nanometers. Visible light (which is just another ... 5 If you are considering human vision there is a definite (and surprisingly small) number of distinguishable colors. This is known as a MacAdam diagram and shows a region around a single color, on a chromaticity diagram, that is indistinguishable from the color at the center. The total number of colors would be the number of ellipses needed to completely fill ... 5 Power consumption is about linear with frequency. The processor contains millions of complementary FETs as shown. When the input goes low the small capacitance gets charged and it will hold a small amount of energy. A same amount is lost during the charging. When the input goes high again the charge will be drained to ground and be lost. So with each ... 5 Because the frequency and intensity are not related to that extent (by which you got yourself confused). Frequency gives the number of vibrations per second (and so, the unit s^{-1}). As @daaxix says, you have used intensity to mean irradiance. Irradiance is the amount of power the wave has delivered per unit area and hence, W/m^2. Both are quite similar ... 5 The light intensity I is the power P, that irradiates an area A, I=P/A, therefore the units \mathrm{W}\mathrm{m}^{-2}. This means, that if you have two beams with equal intensities, the area is the same and the power has to be the same as well. You are probably confused by the relation between frequency and energy of a photon, E_\nu = h\nu. But ... 5 Since you used the tag wave-particle-duality, I imagine you mean the frequency f that corresponds to an electron's energy E via Planck's relation,$$E=hf, where $h$ is Planck's constant. That is a valuable question and nothing to get picked on for. After all, if the electron is a wave with wavelength and so on, it surely has a frequency, right? It ...

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