# Tag Info

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Adding two stokes vectors does not give you the stokes vector for the combination of the two beams. For example, adding a beam of horizontal and vertical polarization would make a beam of 45deg (linear) polarization. In order to add two beams you would have to come up with a Muller matrix $M_\vec{a}$ for adding $\vec{x}$ to $\vec{a}$. Unpolarized light has ...

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Carrier wavelengths close to the visible part of the spectrum are used in optical fibre links. The precise wavelengths are chosen to minimise attenuation in the fibre. But as Ross points out, the bandwidth is then limited by the modulation/demodulation electronics.

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There are two basic problems with visible light communications: (1) atmospheric absorption and scattering, (2) antenna directivity/beam width and its size relative to wavelength. The absorption is easier to understand, which is insignificant below 10GHz, and then progressively increases with huge discrete lines, see ...

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To have a communications link, you need emission, transmission, and reception. For transmission, there are well measured windows in the atmosphere. The atmosphere transmits visible very well, and some lower frequencies, but X-ray and gamma not so well, so those are out. For emission, you need some way to modulate the signal, which so far has meant ...

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No, because black is not an "actual" color, but the absence of light. In other words, when you perceive a color it is because there is some electomagentic radiation that is bounced back from the object. Our retinas get exited and interptet that as a color (they interpret different colors in dependence of what frequencies are reflected). But our brain ...

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The problem is that the shorter wavelengths get absorbed by anything in the middle, but micro and radiowaves (depending on the freqency), are either transparent or bounce back from the upper athmosphere. The highest frequencies (such as x rays) can pass trough objects, but they are harmful to us.

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This is misconception that light is some kind of 'mix' of waves and particles. Instead, It actually IS both waves and particles at the same time, you can't separate them from each other. So probably, the answer could be: you see particles as well as waves.

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The apparent color of an object does change when different frequencies of light are incident. In the extreme case that light of a single frequency is incident, only that frequency is reflected, and the color of the object ... and every object ... is the color of the incident light. The apparent color of an object changes under different types of "white ...

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The phenomenon is a little different that you describe. A red object, for instance, absorbs most of the frequecies and only reflects back the ones on the red spectrum. Actually the reason that a red object looks black on blue light (which has higher frequency) is that the blue light will be absorved by the object (not reemited), and there is no red light to ...

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It would be physically impossible to be able to "see" light as anything other than a particle (photon). The only time photons, or any other subatomic particle for that matter, can be described as a wave is when we are NOT looking at them.

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You are seeing particles. However there's more to this than meets the eye so I need to explain exactly what I mean by this. Light is neither a particle nor a wave. Instead it is a quantum field. As a general rule while light is travelling it appears as a wave, but when the light quantum field is exchanging energy with anything it does so in quanta that ...

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Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels Both the speed of sound and the speed of light have fixed values regardless of the speed of their source Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal. You can measure acoustic ...

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It does follow the same logic. There are some barely seen mini rainbows in the outer layers called supernumeraries. The red on the supernumerary overlaps some of the blue on the original.

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Acoustic Wave is a wave in which motion of one atom causes motion of another atom because it is lying next to it. Light is change in electric or magnetic field which further causes changing fields.

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Acoustic waves need a medium through which to travel. Light does not.

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Acoustic waves are longitudinal waves. Light is a transversal wave, hence not an acoustic wave.

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I would like to point out this video by minutephysics: https://www.youtube.com/watch?v=R5P6O0pDyMU so far the better and simpler explanation of the color of the sky. It takes into account also color theory (not particle physics of course) and not just scattering.

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I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow. See the sun is producing a spectrum of electromagnetic waves, the characteristics of which depends on the Surface temperature of the Sun. Moreover you can observe from the spectrum that the emitted light's ...

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It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? Photons have zero rest mass so when they are at rest they have no mass. They are never at rest so this is a little misleading. And if photons do have some effective mass while traveling at speed of light then only can a black hole's ...

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Indeed photons are massless particles, so they follow "quickest paths" during their propagation in space-time; these are called "geodesics". However, general relativity doesn't simply says how the way masses attract each other is modified, it most of all says that mass (and energy density) curve space-time; this also curves the geodesics, which photons (in ...

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One answer lies in utility and the evolutionary origin of our eyesight. Our eye is basically the same as that of a fish. Water absorbs red light. It absorbs near infrared even more strongly. Fish didn't evolve the ability to see in the near infrared because this strong absorption would make that capability rather useless. Near infrared would also be rather ...

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Molecules of air are all around us all the time. If so, during daylight do rays from the sun diffract as it passes through molecules in the air? and if so is this diffraction negligible to be noticed? plus does this affect anything? While both diffraction and scattering refer to redirection, I think scattering is the better term here. The molecules in ...

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Re: diffraction of light by molecules in the air. The distances between atoms in molecules is approximately $0.1 nm$ which is about $5000$ times smaller than the wavelength of visible light. For diffraction effects to occur the structure should have similar dimensions to the wavelength of light - in this case the molecules are just too small for ...

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This looks like an interference pattern like the type created by a diffraction grating. Note that to the left and right there are little rainbows with the red outermost and then green and blue innermost. Also the further away from the centre the wider the rainbows get. These features are characteristic of diffraction grating type phenomena. Now here the ...

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I get that the sun is producing white light which is scattered threw our atmosphere so that the light of the sun reaching our eyes is yellow. Not very much. When the sun is high in the sky, most would describe the light as "white", not "yellow". That would be more true for a sun low to the horizon. So how come if I look to a piece of white paper ...

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When the sunlight is colored, for example by the atmosphere, the "white" surface has to be slightly colored with the complementary color to looks like white or light gray.

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The wiki article you quote is succinct, the photon is an elementary particle in the table of elementary particles of the standard model of particle physics. It is a quantum mechanical entity which means it is described by a wavefunction whose square gives the probability of finding the photon at (x,y,z) at time t. The double slit experiment with a single ...

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"Vantablack, made out of carbon nanotubes, is designed by Surrey NanoSystems and absorbs 99.96% of all light that hits it. Conventional black, such as black paint or fabric, absorbs between 95% and 98% of light."

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As I indicated in my comment on Rod's answer, some very powerful lasers do exist - and while their photons don't have much momentum, they do "pack a mean punch". In fact, laser ablation (where a laser beam produces significant local heating and material is ejected at high speed) may just produce the phenomenon needed. Let's take the example of the LLNL ...

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Generally, there are only two things that can happen in the hypothetical experiment you describe. Either photons are emitted and absorbed in some place (detector, mirror, anything), or they are not emitted (and of course not absorbed). Both cases conserve energy. The latter possibility is the one that is somewhat counter-intuitive but it goes to the heart ...

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Supposing we could shield ourselves with a perfectly nonabsorbing, reflective shield so that light would perfectly elastically bounce off us, thus preventing high power beams from incinerating us as Anna V's answer validly argues they would. Then the "fundamental" answer to your question is "because light has zero rest mass"; to explain further: the ...

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A ray of light is a geometrical line describing the propagation of an electromagnetic wave. The electromagnetic wave is composed of zillions of photons each with a tiny momentum. The momentum is not large enough to sense an impact, it is pico newtons even for a laser beam. Lasers can have very high energy and momentum, but like knives, they cut soft tissue ...

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The IR sensors that OP is talking about work by emitting IR from an LED and then measuring IR intensity reflected back from an object close enough (scroll to Sharp GP2Y0A21YK IR Proximity Sensor). Addressing the comments: the temperature of the robot and the resulting blackbody emission in the IR is small enough to be irrelevant to the question. IR is used ...

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The only requirement for light is that the electric field must be perpendicular to the magnetic field at any given point in time or space. This assumption arises naturally from Maxwell's equations. The most intuitive way of thinking about light is with the picture you included of the light wave. However, you have to imagine an infinite number of light waves ...

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The book is imprecise because there are other types of polarized light. Now, consider a point in the path of the wave, like the green point in animation you mentioned. In that image, the light is linearly polarized so the electric field draws a line at the location of the point. However, there is also circularly polarized light where the electric field draws ...

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See Wikipedia "Unpolarized and partially polarized light". It's written there "Most common sources of visible light, including thermal (black body) radiation and fluorescence (but not lasers), produce light described as "incoherent". Radiation is produced independently by a large number of atoms or molecules whose emissions are uncorrelated and generally of ...

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One way to conceptualize "twisted light" is to think of a ball being thrown in the air. Usually you spin the ball slightly as you throw it, well imagine spinning the ball a specific amount to encode some information. Then you could build a ball cannon that sets the amount of spin for each ball it fires, that encodes information on each ball. For a binary ...

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Since this was not stated yet, I would just like to give my stance on it. All fundamental particles can be seen as excitations of fields. This is true for photons, electrons, neutrinos, etc. Do these fields need a medium in which they propagate? Not as far as we can tell. Everything we see and experience are excitations of these fields, a single one of ...

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It may be useful to start this explanation from the origin of a light wave: an oscillating charge. Start with the idea that a stationary charge is surrounded by an electric field, then imagine wiggling that charge up and down. Now the field lines will turn to wiggles instead of straight lines. Those wiggling field lines are the electromagnetic waves we call ...

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Einstein once compared the photon with a famous person (sorry I forgot the name) who changed confession at young age and returned to its initial confession before he died: Light behaves as a photon at the starting point and at the end point, and it behaves like a wave during its travel. By the way, the light wave is not going up and down, it is not a ...

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The light we see with our eyes is electromagnetic radiation, very well modeled by Maxwell's equations. Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. Note that the electric and magnetic fields ...

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Light waves are exactly a theoretical explanation of light radiation. Propagation of waves of electromagnetic fields is a good theory that works for low frequencies, but as Einstein showed (and was Nobel prized for) the photoelectric effect can only be explained if electromagnetic radiation is emitted as directed quanta of energy. I guess that experienced ...

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Macbooks use LCD screens. These have a liquid crystal layer with a backlight behind it. The LCD screen works by changing its opacity i.e. it controls the amount of light that can be transmitted through it from the backlight. If the LCD is not blocking any transmitted light then all the light from the backlight is transmitted and the screen looks white. If ...

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It is common sense to mix phenomenons electromagnetic radiation and radio waves. EM radiation is the result of electron acceleration. Then higher the acceleration then higher the photon's frequency. Different from this the frequency of radio waves is the modulation of electron acceleration made from the antenna generator. But at THE end antenna emit photons ...

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This is a reasonable enough question. Imagine that you are looking at the object and behind you, you have a very large mass of clear, but very high index of refraction material (with index of refraction $n$). Then you could easily say "I wonder how this looked 100 years ago", and then very quickly run behind your large block of material at some speed \$c/n ...

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But the theory of relativity tells us that you need a lot of energy to reach the speed of light and when you won't be able to travel at the speed of light no question arises of looking back in time

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The speed of light can't be measured anymore (in SI units) because it has had a defined value since 1983. See Why do universal constants have the values they do? Before 1983, the meter was defined in terms of the wavelength of a certain emission line of krypton 86. The second is also defined in terms of an atomic standard (the frequency of a transition in ...

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I'm going to take a different tack on this question. The use of RGB in computer monitors and other color display systems is entirely determined by human biology, not physics. The subpixel values of R, G, and B represent the amount of activation induced in a human's cone retinal cells. Monochromatic yellow light (about 570 nm wavelength) creates signals in ...

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To display colors on monitors, programs needed some way to define the color. Since most monitors (at the time) were CRTs with RGB guns, the easiest method was to specify the intensity of the RGB guns. Each gun was given a byte (255 levels) of intensity, although I believe that the hardware could not resolve the smallest changes. So instead of 256 levels of ...

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