# Tag Info

16

The problem is that you are confusing light intensity with energy of a single photon. The photoelectric effect requires a certain energy per photon to work. But low light intensity just means fewer photons come - you can actually see the grain if the conditions are too dark: every pixel can get ~10 photons or less... and yet still, each photon that comes has ...

8

The total confusion comes from mixing classical concepts, light, with quantum mechanical ones, photons. The paper just demonstrated that light can move slower in vacuum if manipulated optically before. A light beam is composed out of zillions of photons , and its properties are emergent, are built up, from the wavefunctions of individual photons. What the ...

7

Yes, but you'll have to go really, really fast. And even then, don't worry about the photons. The relation between velocity $v$ and the observed and "true" wavelength $\lambda_\mathrm{obs}$ and $\lambda$ of the light is $$\lambda_\mathrm{obs} = \sqrt{\frac{1-v/c}{1+v/c}} \lambda.$$ If you consider optical (i.e. visible to humans) light with a wavelength ...

6

There are several different things that need to be explained / explored here. First - the speed of light in vacuum is independent of frequency / wavelength. The same is not necessarily true for light in any medium other than vacuum: this is why we can see rainbows! Second - not all objects emit "white" light. The emission spectrum of a star depends, among ...

5

It is possible and it has already been observed. In the following article they suggest both single and double step emission mechanisms: Two Electron Photoemission in SolidsR. Herrmann, S. Samarin, H. Schwabe, and J. Kirschner Phys. Rev. Lett. 81, 2148 – Published 7 September 1998 The single step emission process is because of correlletions of the two ...

3

What happens to photons when they hit our eye? Some of them pass through the iris and are focussed by the lens onto the retina where they are absorbed by rods or cones. where do they end up? Some of them end up absorbed by rods/cones, some by other tissues, some are reflected (c.f. red-eye in photography). why our eye don't get overheated? ...

3

When people say that photons always travel the speed of light, they meant in terms of reference frames. And in fact, even through water, photons do travel the speed of light. The only reason why they appear to travel more slowly is because they "run into" molecules, and the path is not straight. In a vacuum, there are no molecules to hinder the light, and it ...

3

The electromagnetic potential $A^\mu$ is a four-vector, and hence transforms in the fundamental representation of $\mathrm{SO}(1,3)$, i.e. $A^\mu\mapsto \Lambda^\mu_\nu A^\nu$ where $\Lambda$ is the usual 4x4 matrix associated to a Lorentz transformation. Your question seems fundamentally confused about the difference between the field and the particle. The ...

3

Absolutely it can - and it happens all the time. If you excite an atom, it can go through various "stages" of decay back to the ground state - with each drop in energy resulting in an emission of radiation. This happens during photosynthesis: see this page from which I copy this image: As you can see, there are multiple paths for the energy to be lost by ...

3

The electron-positron pair has a center-of-mass reference frame where the momentum is 0. Obviously, there exists no one-photon system with positive energy which has 0 momentum, as the energy-momentum relation for a photon is $E = p c$.

2

The trick is that because the "slit" is infinitely wide, you shouldn't work in the far-field approximation (Fraunhofer difraction integral which leads to fourier optics), but with distances computed to the square order (Fresnel diffraction integral). The results are appropriately named Fresnel integrals (this time this is a name of a special analytical ...

2

The formula E=h*f is a quantum mechanical formula for the elementary particle called a photon. The photon travels with velocity c in vacuum. If it is not a vacuum it will interact with appropriate quantum mechanical interactions, and in the interval between interactions it will be traveling with velocity c. If its interaction is elastic, as happens in ...

2

From the theory of Thompson scattering (see http://farside.ph.utexas.edu/teaching/em/lectures/node96.html ) we know that a charged particle of mass m interacting with a plane wave electromagnetic field of Strength $E_0$ and frequency $\omega$ has an effective dipole moment of magnitude $$d=\frac{e^2E_0}{m\omega^2}$$ Note that the dipole moment scales ...

2

There is light all around the lamp, and all around you (except of course for the points that light cannot reach because of walls or whatever). You just only perceive the light that comes into your eyes. The light that comes directly from the lamp is generally more intense than that reflected by the objects around you, so that you tend to ignore the latter. ...

2

Your question is the one that Einstein pondered for long time and from which Special Theory of Relativity was born. He wondered what could happen if you travel at the speed of light how would you see a ray light. The problem was that according to Maxwell's Electrodynamics, explained light as oscillating $E$ and $B$ vectors along space and time, so as a ...

2

Think about it this way. If you had a radioactive atom it might decay and emit a photon. And you don't know when. So there is a whole range of times for when a nearby photon detector might go off. But there is also an issue of coherence. Which is a specific technical term, not a word people use for emotional purposes. If you have a short coherence length ...

2

Building on a comment by CuriousOne (who, honestly, should leave off commenting since he only ever writes answers in then anyway): A photon is not an object in and of itself. A photon is an excitation in a quantum field, which is not localized but fills space. In the double slit experiment you have an emitting source, a mask with two slits, and a ...

2

Light does not travel at the same speed in all mediums. In water the speed of light is less than that in vacuum.

2

There is no stimulated emission in an FEL. The SASE (Self-attuned spontaneous emission) process works as follows: A bunch of electrons is accelerated. The electrons pass a first undulator, creating synchrotron-source type light. Light and electrons free-stream together, since both move at the speed of light, there is a back-reaction of the light on the ...

2

If you measure distance in units of the Schwarzschild radius, then black holes look identical regardless of mass or "size" (as you can verify by inspecting the metric line element). Therefore the answer is negative: A smaller black hole looks exactly like a big one does from further away. Also, the photon sphere is a shell which completely encloses the ...

2

This link summarizes the measurements of the speed of light. The first measurement of c that didn't make use of the heavens was by Armand Fizeau in 1849. He used a beam of light reflected from a mirror 8 km away. The beam was aimed at the teeth of a rapidly spinning wheel. The speed of the wheel was increased until its motion was such that the light's ...

2

Similarly the reverse is possible where a photon can spontaneously morph into two equal particles of matter and antimatter. Wrong, a photon has to interact with some other particle or field in order, if it has enough energy , to create a particle antiparticle pair. This is because of special relativity: the photon has a mass zero whereas the particle ...

2

In a static spacetime, there is (by definition) a timelike Killing vector field $\xi^\mu$, which implies that geodesics with four-velocity $u^\nu$ have a conserved quantity $\epsilon = -g_{\mu\nu}\xi^\mu u^\nu$. For example, in Schwarzschild spacetime, this is $$\epsilon = \left(1-\frac{2M}{r}\right)\frac{\mathrm{d}t}{\mathrm{d}\lambda}\text{,}$$ where ...

2

What we call reflection is in reality a more complicated process than bouncing a ball to a wall. For the part of the electromagnetic radiation that we call visual light and for low densities of this light the surface electrons are responsible for the absorption and re-emission of this photons. So yes, mirror will gain momentum and the photons will lose ...

1

For the theoretical background with an occasional nod to experiments, my bible is "Optical Coherence and Quantum Optics" by Mandel and Wolf. This book is both a text book and a reference for researchers. It covers the basics of random signals, quantum mechanics, the quantum theory of radiation, quantum optics, a bit of nonlinear optics, a bit of laser ...

1

As I understood two photons will not interact with each other to produce interference pattern rather one photon behaves like a wave near two slits and go through both holes at same time. It looks like particle is spread in space and behave like a wave and will go through both holes at the same time to produce interference pattern.

1

In one word the answer is interference. Now more details. Assume you are traveling through something which has refractive index different then 1, like water. It actually means that for "slightly" different frequency of light water molecules will start to absorb the photons, i.e. shake together with electric field. However if you stay away from that ...

1

What bugs you here is that the word "speed" isn't precise at all when we talk about a wave in a dispersive medium, such as water. What we usually refer to when we say the "speed of light" is the constant we call $c = 299792458$ m/s, which is equal to the velocity of light in the vacuum. Vacuum is non-dispersive, so the group and phase velocity are the same, ...

1

We don't harvest waste artificial light, because it would be ridiculously expensive to do so. The energy in sunlight is, at full sun, $1000W/m^2$. That's way higher than any artificial light in normal circumstances. So it's far more economic to position solar panels to optimise collection of daylight, rather than to capture artificial light at night. Any ...

1

let's assume we have 2 rays moving at 180°. If the container ( and the background ) move instead of the photons, the container would have to move in the direction of both rays. But, it's impossible since we assumed 2 antiparallel rays. How can container move forward and backward simultaneously.

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