New answers tagged

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Telecommunication networks use this technique in the optical fibre network, frequency division multiplexing (FDM) don't worry about pedants trying to score points trying to make you revise your question. It's true you have mixed up your units and there are many different forms of modulation. The simplest is probably AM and that will give you an upper and ...


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To remove added complication from the problem. consider a front surface mirror, silver coating on the front. Maxwell wave theory simplifies the problem. A plane wave impinging on the metal surface, induces a current into the surface, that then generates a wave in the opposite direction, as a matter of interest the magnetic field component of the incident ...


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I understand that EM waves are self-perpetuated due to the interactions between changing electric and magnetic fields as described by Maxwell's third and fourth equations, but I'm stuck conceptually on what they are if there are no electrons or conductors around. people were stuck like you begininning of the 20th century thinking the EM waves required a ...


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You've come to a long time phyilosophical question: does a field actually "exist"? We cannot measure fields. There is no instrument able to measure a "field". The instrument you use to measure electric field is actually based on forces. The instrument detects an electric force and then it deduces the value of the $E$ field. In an empty ...


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The emissions and absorptions of the molecules are discrete. What makes the black body spectrum continuous is the projection of random directions of an arbitrary number n of molecules to the direction of observation. This is done with the Boltzmann statistics. Here a more detailed picture: The Planck law gives the number of photons in the direction of the ...


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Have you ever wondered why things show colour? Well when light is incident on a surface, light of certain wavelengths are absorbed and we see the complement colours.For example a green surface absorbs light of all colours other than green. But a white surface is a surface that reflects out light of all wavelengths unlike a black surface which absorbs all ...


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Your question is posing a characterization that's grounded in a misunderstanding and does not fit the situation at all. What you described is not what spin is, nor how it is characterized. And, in fact, some of the replies are also grounded in a misunderstanding and mischaracterization. First, photons don't have spin. They have helicity. Spin is an attribute ...


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I assume the photon moves in $z$-direction here: The two polarizations of the photon (corresponding to its two spin states) are not linear polarizations, but clockwise and counter-clockwise circular polarizations (you obtain linear polarization as a superposition of a clockwise and counter-clockwise one). These are completely rotationally invariant. The spin ...


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If you rotate a polarized classical electromagnetic wave by 180° about its axis of propagation, you don't get the same wave back; you get the same wave phase-shifted by half a cycle. You need to make a full rotation of 360° to get the same classical wave solution back, so by your logic, the photon should indeed have spin 1. This is contrast to a ...


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The photons are point quantum mechanical elementary particles of spin either +1 or -1, to their direction of motion. Their mass is zero and their energy equal to $hν$ where $ν$ is the frequency of the classical wave built up by thousands at least of photons. The way classical electromagnetic polarized waves are built up by their constituent photons is seen ...


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How flash which is just light can send information? I know that you can send information with fiber optics but I don't think that this is the same. I think it is (more or less) the same. Maybe this question is no so much physics as it is information theory or telecommunication. Very much simplified: most telecommunication uses a carrier wave and an envelope....


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So who is correct here? Are CCD cameras (and probably others) actually sensitive the infra red light? Or not? Point a TV remote at your camera so you can see it's image, and then push one of the buttons. You should see the infra red LED flash the signal that button sends to the TV. (It's also how you can see that the batteries in the remote are good.)


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The reason is that LED lights typically pulse rather than give continuos light. This can be because of flickering due to converting AC power into DC, but also dimmer or driver circuits regulating how bright they are by only keeping them lit for a fraction of time (but, in high quality systems, flickering is so fast viewers will not notice). When you move the ...


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The speed of light is 299 792 458 meters per second, so in 1 nano second, light travels about 30 centimetres. So with a led and a frequency emitter generating a 0.5 10-9 square we can detect on the other side of the bench with a photo sensor the fall of the signal and adapt the distance until it would be in phase with the emitter and we can measure +-30 cm ...


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Normal human eye has 4 photoreceptor types. Each of them has a specific response to certain electromagnetic frequencies/photons energy. 3 of them are named cones and are more sensitive to only one part of the visible light, making them more able to dissociate colors. The other receptor type is rods, which are more sensitive to light so better for night ...


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Interesting thought aside from loses on reflection as Eric pointed out. But there are a couple other problems, too. Suppose you have a lot of light bouncing around inside. Suppose you open a door so some can get out. How do you allow only a little out at a time at a controlled rate? One problem with batteries is they concentrate a lot of energy in a small ...


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A few things you need to consider: (1) You won't be able to put an arbitrary amount of light into the box -- at some point the pressure of the light inside will be greater than the pressure of the incoming light, and then no more will get in. (2) A far bigger issue: in the real world no reflecting surface is 100% efficient, and the light is going to be ...


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You can see this from the Lensmaker's equation for thin lenses $$\frac{1}{f} = \frac{n'-n}{n} \left( \frac{1}{R_1} - \frac{1}{R_2} \right)$$ $R_1,R_2$ are the curvature radii of the surfaces of the lens. $n^\prime$ is the refractive index of the interior of the lens, whereas $n$ is the refractive index of the medium outside the lens. So for $n\approx 1$ (...


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You forgot garden-variety absorption! Here, light promotes electrons from lower energy states to higher energy states. However, skin is made of many small particles, so scattering is important as well. Here’s the mental video: Sunlight illuminates the skin. The light encounters a disordered dielectric interface consisting of mostly transparent wavelength-...


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The simplest way to approach this is to consider a photon (or a laser, with identical photons), and then Lorentz transform the 4-momenta. But first, since you did not name the reference frames (which makes answering easier): The inertial frame of the rocket (observer) is henceforth $A$ ($O$). Moreover, "up" is $z$, and the beam is shone in the $x$ ...


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The movement is relative. So the reasoning is the same as if we consider the spaceship at rest and shining a laser. In this case it is the observer that is travelling close to the light speed in relation to the ship. And of course the beam will be diagonal to him.


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There are two main ways to understand this: visible light If you are asking about visible light, then the chair is not a primary source of light (except for very special circumstances, like a wooden chair being on fire, or a metal chair being extremely hot glowing), but in normal cases the chair is only reflecting the light that is shining on it from a ...


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The total speed of the light beam will be $c$ (the speed of light is always the same) but the direction of the light will, obviously, depend on the observer. If the light is shining perpendicular to the axis of the spaceship, then the "forward" motion of the light (the component in the direction of travel) will be the same as the forward velocity ...


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If you talk of light as the entire Electromagnetic Spectrum, then YES, it is a primary source of "light" as all objects in this Universe emit some form of electromagnetic radiation (as long as they are not perfect black bodies). But by "light", if you mean the "visible spectrum", then NO, a chair does not emit light.


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A chair can be a source of light. It could even be your only source of light if all other directions are blocked off. Light comes from energized electrons emitting photons. When a light bulb energizes the electrons emit photons that radiate out to the chair and energies electrons on the chair which also emit photons to your eyes. Both situations emit ...


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"Chair is a source of light" is an English phrase. Like many phrases, it has many meanings, and you have to ensure that the reader understands what you mean. Niels points out that, for virtually everyone, "chair is a source of light" is considered to be a false statement because we typically are only thinking of primary sources. However,...


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The speed of light is constant. Photons only travel one speed and one direction forward, so there is no sideways. You cannot add or subtract the speed of light from the speed of the space ship. The best way to think of it is to picture the energy being carried away from the light source as apposed to being emitted or launched. So if you have a light source ...


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For an observer at the surface of earth, close to the point where the beam is tangent to the surface, the equivalent principle is valid. The measured deflection is the same as it would happen in a rocket in space with an acceleration $g$. But for an observer very far from earth, the total deflection is $2 \times$ the calculated considering an acceleration of ...


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Yes it will curve, but not at 9.8 m/s$^2$ as predicted by Newton's theory. Its curvature will be twice that value as predicted by General Relativity.


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Yes the light beam would curve at 9.8 m/s2, as per Newton's theory: To see why a deflection of light would be expected, consider Figure 2-17, which shows a beam of light entering an accelerating compartment. Successive positions of the compartment are shown at equal time intervals. Because the compartment is accelerating, the distance it moves in each ...


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The beam of light would curve under the influence of gravity, because light goes as straight as it can through curved space. Calculating the exact amount of curvature is relatively complex, given the Einstein equations.


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First of all, color is not reflected. When light is reflected it creates glare which is an image of the light source itself. Color is created either when (1) light of a particular wavelength is emitted from a light source or (2) is absorbed and then re-emitted by a physical object. So, when we say an apple is red for example, it refers to the second case. ...


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If the entire screen of a phone is set to white, the screen 'appears' approximately as a diffuse light source. But each individual pixel acts as a point source - i.e. if one pixel is set to white and the rest of the screen is black, then the lit pixel acts approximately like a point source - radiating light in all directions. So you could replace that pixel ...


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It is important to specify how the travel time is to be measured. If it's measured by an observer traveling along with the light - at the speed of light - then the observer will not experience time passing at all. He and the light will travel to anywhere in the universe in what he measures as no time. To an observer in any other inertial frame (that is, ...


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To the question: No. One light particle (photon) cannot be split up. The room is lightened up completely due to different photons, which come from a source that is always somewhat divergent. This means that the photons come at different angles and are therefor reflected at different angles, lighting up the room. To the example: The reason for what you ...


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Remember that light is essentially a wave, like sound (yes yes particle-wave duality, but we're not getting to that today). When waves pass through a gap in an obstacle, it diffracts and spreads out. The angle at which the first minimum intensity occurs depends on the wavelength. For me singing "ahhhh" at 160Hz, the wavelength is 2.1m, but for ...


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What do we see with our eyeballs? As one commenter has replied, what you see is neither particles or waves, but the qualia of light, that is colour - whether that be green, red, orange or just white. Ontology is complex and is not just one thing and it is scale dependent. For example, in the situations you have described, classical physics is sufficient. It'...


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Light is a quantum level effect. The worst thing that science ever did, was call these things particles. The Greeks proved a long time ago, that the smallest particle in existence is the atom, which is true. That is not because the Greeks didn't know light, or didn't understand there had to be a way to get smaller, but because they understood that below the ...


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Well, theoretically, anything (including light) has to take infinite time like any other thing in existence to travel an infinite distance! Although, if it is ONLY light in this universe of yours, time wouldn't exist because theoretically it needs matter to be experienced! I hope the first sentence answers your question. Also, if it's D/t then it's still ...


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This is the simplistic answer that I found useful when I was a kid. It isn't strictly rigourous, but it was useful. (See also "All models are wrong, but some models are useful".) Light is something (a "thing", a phenomenom, whatever) which has the property that: if you do a test that asks "Does this thing behave in this way, which ...


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How is this Possible? It isn’t. $1+2+3+4+...\ne-1/12$. Although this mathematical mistake is undeservedly famous it is nevertheless a mistake. The Riemann zeta function $$\zeta(s=\sigma +i t) = \Sigma_{n=1}^{\infty} n^{-s}$$ for $\sigma>1$. Also, $\zeta(-1)=-1/12$. But since $-1 \ngtr 1$ it is clear that $$\zeta(-1)\ne\Sigma_{n=1}^{\infty} n^1=1+2+3+4+......


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It's kind of a funny misconception that the sun is yellow. I mean, astronomically speaking it is indeed a yellow star, more precisely G-type main sequence / yellow dwarf... but don't be fooled by the terminology: astronomically speaking, you'll also find that the Earth consists completely of metal! Actually you should consider the sun as white. The main ...


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Light is a range of frequency of electromagnetic waves which our eyes can detect. Light consists of photons, which is a weightless particle. Light is a medium through which energy can be released.


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The perceived color of sunlight depends on the angle at which you are viewing it, and it's decided by the time in a day. At noon it looks white, but when it's closer to sunrise or sunset it looks more yellow. However, just because it looks more yellowish to you does not mean it's a purely yellow light, it's never monochromatic. In the yellow sunlight, the ...


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An isolated single atom is small compared with light wavelength and so ”scatters” light in all directions if at all. Also one atom cannot normally know which way the “normal” to the mirror is. However Interference of all the scattered waves from the different surface atoms will be constructive in the specular reflection direction. This is largely independent ...


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What is light? And how do we know that light is an electromagnetic wave? Light is what is needed for our eyes to see, and until the coming of Maxwell, light and electricity and magnetism were two very different physics observations. To start with electricity was known from rubbing stuff together and getting sparks. Magnetism was known from stones found in ...


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It is because the sum of all possible light colors is perceived by your eyes as white, so a beam of white light has all those different colors mixed together in it. The prism is a convenient way of separating out the different colors so we can see them individually. By the way, once you have separated the colors in this way, you can recombine them with a ...


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Your instructor is wrong, and you are right. Placing a compass needle in a light beam will not cause the needle to turn, the way it certainly will if you place that compass near a magnet or a piece of wire carrying an electrical current. This is because light beams do not generate electrical current flow or magnetic fields in space, to which a compass could ...


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Electromagnetic waves are solutions of the wave equation that we derive from Maxwell's equations. When we use Maxwell's equation to compute predictions of what we expect to see when making experimental observations with light, we see that the predictions agree with the observation. Therefore, we conclude that light is an electromagnetic wave. The frequency ...


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Sunlight is "white". Blue sky is due to Rayleigh scattering, where the intensity of scattered light depends on the fourth power of the frequency, then this is why the sky is blue, since blue is in the upper bound of the visible spectrum in frequency. This is because the molecules in atmosphere have a size much smaller than the wavelength of light. ...


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