# Tag Info

117

A tin foil hat can block: alpha rays electromagnetic waves, where the wavelength is short enough to not diffract around the edges (counterexample: FM radio waves), but not short enough to punch through the foil (counterexample: gamma rays) ultrasound rain job offers marriage proposals But, then again...

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Can tin foil hats actually block anything? Anything? Sure. As already noted by Daniel Griscom's answer, tin foil can block several "things" including rain, alpha rays, and electromagnetic radiation with small enough wave length that the radiation cannot diffract around the edges of the hat. If they can, what frequencies? Since you mention frequency ...

21

It's spherical because the main dish cannot be steered; steering is done by moving the receiver (the big thing hanging over the center of the reflector). A parabolic reflector would produce varying errors when aimed in different directions; a spherical reflector has the same error for all directions. Presumably the receiver is designed to compensate for ...

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In the early days of radio, the resonance of the antenna in combination with its associated inductive and capacitive properties was indeed the item which "dialed in" the frequency you wanted to listen to. You didn't actually change the length of the antenna, but by changing the inductor (a coil) or capacitor connected to the antenna you tuned the resonance. ...

15

In NMR, the strong magnets set the frequency of the nuclear resonance, using the constant magnetic field. Typically the resonance radio waves are around the MHz frequencies whereas Wi-Fi is around 2.5GHz. When the frequencies are different, they don't disrupt each other's signals. Electro-magnets wouldn't interfere, as it is a constant field produced ...

12

You might want to have a look at Does light induce an electric current in a conductor?. It's probably impossible for a radio aerial to emit visible light as the frequency of light is around the plasma frequency of the metal that the aerial is made of. We're not really supposed to address hypothetical questions, but if you could find some material with a ...

11

Did you read the Wikipedia article? It explains the signal rather well, I think. At any rate, it is called the Wow! signal because, as the picture shows someone wrote Wow! in the margin. As for the code and why they were excited, I quote the Wikipedia article, The circled alphanumeric code 6EQUJ5 describes the intensity variation of the signal. A ...

10

They heat it, by different degrees depending on the polarization of molecules in the tissues and liquids. The molecules try to re-align after the radio-wave field and the movement dissipates as general heat. Think microwaves.. a consumer-grade microwave oven operates at the same radio wave spectrum as your home WiFi network (2.4 GHz) but much stronger. The ...

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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 radio ...

10

A human body may reflect and absorb radio frequencies, though not very efficiently. It may as well act as a resonance chamber for certain frequencies. For a signal of 100 MHz, the involved wavelength is 3 m, and so it is possible that parts of your body are acting slightly as a resonant chamber. (for an optimal resonance, you should have 1.5 m diameter, too ...

9

Radio waves are large wavelength waves, and non metal walls are transparent to the radiation at those wavelengths, depending on the thickness of the walls, because there are no energy level "receptors" to absorb them in bulk by excitation of electronic orbits. The wavelengths start from a centimetre up to kilometre, which is the width of the cycle of the ...

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The answer to your question is Yes. More specifically, light is 400 to 800 Teraherz, RF is 3 Kiloherz to 300 Gigaherz.

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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 ...

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Yes, LED's, luminescence of phosphorous (CRT screens), most of the current screen technology, fluorescent lights. They aren't a thermal source. They heat up due to the electric current but that's not the working principle.

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This is something we must all have observed, but I don't know of any definitive study. In the absence of hard data I can think of three potentially relevant effects: Dielectrics, like the human body, deform electromagnetic fields in their vicinity The wavelength of FM radio is around 3m and therefore comparable to the size of a typical human. This means ...

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From a physics perspective, the fundamental reason for this is something called the bandwidth theorem (and also the Fourier limit, bandwidth limit, and even the Heisenberg uncertainty principle). In essence, it says that the bandwidth $\Delta\omega$ of a pulse of signal and its duration $\Delta t$ are related: $$\Delta\omega\,\Delta t\gtrsim 2\pi.$$ A ...

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The idea behind the quarter wavelength antenna is that it is self-resonant: it is "tuned". You can however use an antenna of any size to pick off some electromagnetic energy - and you can tune the antenna by adding some inductance in series (or inductance and capacitance). The reason that you tune an antenna is simply this: you want it to have real impedance,...

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Let your carrier signal be $A_0 \cdot \cos(\omega_c t)$ with amplitude $A_0$ and carrier frequency $\omega_c$. Let your signal be a simple wave, $\phi(t) = A_s \cdot \cos(\omega_s t)$. Then the modulated signal becomes $$A_0 A_s \cdot \cos(\omega_c t) \cdot \cos(\omega_s t)$$. In addition, as pointed out by George in the comments, the carrier also gets ...

7

You're not boosting the signal; you're either acting as a reflector (capturing a bit more of it to feed to the antenna) or blocking a competing source, or perhaps a bit of both. By analogy, when you hold your hand to your ear to help you hear something, your hand is acting a reflector for sound waves to direct a little more energy into your ear. It can also ...

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Microwave ovens work at a frequency of 2.45 GHz. Their principle of operation is that molecules of fat, or water, absorb this radiation, because they are polar molecules with an electric dipole moment and rotate in response to the stimulus of the electromagnetic field of the microwaves. This absorbed energy is then dispersed throughout the material through ...

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I'd like to point out that blocking UV radiation is useful, as I for one have had sunburn on my head. Foil could be used as an improvised "beenie" hat if stuck out in the open, or as a black-out lining of a hat that is more decorative than opaque (e.g. knit fabrics).

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It depends on what kind of radio telescope you're talking about. If you're talking about a single dish, with say, a horn feed at the focus, it behaves like a regular 'dish antenna' by focussing radio waves to the focus where it is amplified, processed in some manner (such as downconversion), digitised, and so on. A single dish can also have a feed array at ...

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Here's a somewhat technical document talking about insterstellar beacons. A beacon would act as a "searchlight", sweeping across the sky, so that the time spent on each target star would be rather short. With a beam aperture able to illuminate 1% of the sky and working at 0.5 Hz they calculate that 6.9 GW power will be "visible" as far as 6000 light years. ...

6

A guitar string produces harmonics because it vibrates in a non-linear fashion. An electronic oscillator can be made to generate a much purer form of vibration (near sinusoidal) than a mechanical device such as the guitar string. Hence its harmonic level, while not zero, is much lower. For example, the harmonic distortion of a guitar string is probably on ...

6

This isn't hypothetical. There is nothing that a radio does that can't be done in other parts of the spectrum. Many FM/AM radios operate in the optical range too. Your TV remote control uses IR. Lasers are used for high bandwidth point to point communications. And don't forget fiber optics, these are all radios that just use optics for the communication ...

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In the microwave band here are multi-element detectors, but at longer wavelengths the telescope is a single pixel. Yes it does take a while to build up an image, but radio pictures aren't usually very large - not the millions of pixels of an optical/IR image. One big advantage of radio telescopes is that you can combine telescopes 1000s of km apart to ...

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While I agree with Alfred Centauri's answer, I am not sure it gives a direct answer to your specific question for the following reason. If there is a receiving antenna somewhere, there is always some reflection, however minute. If the antenna is connected to a circuit, the conditions of reflection will change, and the transmitter can "notice" that. In ...

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It has nothing to do with the modulation, AM or FM; it is because the wavelength difference. The so-called AM band is between 540kHz and 1600kHz, so its wavelength is about 300m, or so. The FM radio operates in the 88MHz to 108MHz band, or around 3m. The longer 300m EM wave reflects from the gap between the metal in the bridge and the ground (the latter is ...

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For light bulbs and other thermal emitters this is definitely true. Their emission follows the black body spectrum (if you neglect absorption due to the glass container). If you want to be picky: Any device, which is operated above 0 K (which applies to all devices) emit thermal radiation according to their temperature. This is not directly related with the ...

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