# Tag Info

### Why do atoms (iron eg) glow with all frequencies of light when exposed to enough thermal radiation?

They glow when a particle in a higher energy quantum state gets converted into a lower one by the emission of a photon. That is one method of emission. Because individual atoms (and small molecules) ...
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### Can I burn a piece of wood by emitting only one photon per second on it?

No. To set fire to a piece of wood you must deliver heat energy to it at a faster rate than it will dissipate. With visible light, the energy of a single photon arriving per second is negligible. ...
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### What happens to the non-axial photons of a laser cavity?

In short: The part of the light with small enough deviation from the optical axis is kept in the resonator, the rest leaks out, but is stopped by the housing of the laser. Which light is kept? Imagine ...
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### Why do atoms (iron eg) glow with all frequencies of light when exposed to enough thermal radiation?

Matter comes in phases: solid, liquid, gas, plasma Individual atoms/molecules join into lattices when solid, are in collective states in liquid, free in gas, and ionized mostly in plasma. ...
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### What determines if a lamp produces emission or blackbody spectra?

To be a blackbody a source must be in thermal equilibrium and be capable of absorbing all radiation that is incident upon it. The hydrogen discharge lamp can do the former - the hydrogen energy levels ...
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### How is light emitted by an incandescent lamp?

Supplementary answer to the OP's clarifying comment: The main point that still needs clarification, in my opinion, is whether we are really dealing with thermal radiation here, i.e., whether the role ...
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### Can I burn a piece of wood by emitting only one photon per second on it?

Let's try to calculate that: The heat capacity of wood is (Wikipedia): $C=1700 \frac{J}{kg K}$ and the temperature at which wood starts to burn is approx $300^\circ C$. Assuming that the wood has ...
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### Why can Einstein coefficients be derived based on thermodynamically equilibrium relations when they are basically intrinsic?

How can it be that intrinsic microscopic properties (which might be calculated by quantum mechanics on n-level systems including radiation fields) can be derived so "easily" based just on ...
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### How does an electron absorb or emit light?

An atom is nothing but a bounded state of electrons and a positively charged core called nucleus. The electrons in the atom are in bound state and so their energy levels are quantized. Also, it is ...
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### Momentum of photons

Yes in both cases. In fact there is no need to invoke the concept of photons; electromagnetic radiation consistent with Maxwell's equations carries momentum. You might care to search online for ...
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### Is the Sun considered to be a black body as a whole or it is only the photosphere which is a black body?

The photons produced in fusion do not transfer their energy to the photosphere - their mean free paths are less than 1 mm. That is because the solar interior is opaque to its own radiation. There is a ...
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### Why don't absorption and emission lines cancel out in our Sun?

The key point missing from most efforts to answer this question are that the Sun has a temperature gradient with depth. If it were (somehow) isothermal, then indeed the absorption and emission ...
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### How is light emitted by an incandescent lamp?

https://en.wikipedia.org/wiki/Thermal_radiation Depending on the theoretical frame used it may as well be called Bremsstrahlung (free(-ish) electrons in the metal scattering into each other) As far as ...
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### What determines if a lamp produces emission or blackbody spectra?

Imagine an envelope containing mostly vacuum and a little of the gas of your choice and a pair of electrodes. We connect the electrodes to a high voltage supply which contains a current limiter (...
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### Why don't absorption and emission lines cancel out in our Sun?

I think that this is a very good question. In my answer I will only mention the formation of one of the absorption lines, the 589 nm of sodium, and I will call the photon associated with that ...
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### Does each spectral line of an atom/molecule have a unique lineshape?

A lot of questions, but since they are related, we go on: A spectral line is determined by a particular transition in an atom or molecule. In reality, this line isn't infinitely sharp, but has a ...
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### What happens to the non-axial photons of a laser cavity?

Some hit the side wall of the cavity and are absorbed. Some hit the side wall of the cavity and go through. Most lasers are in an enclosure, and the light is absorbed in the walls of the enclosure. ...
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### Photons of a radio wave

We should start with the obligatory warning that a beam of EM radiation is not a swarm of photons. See What is the relation between electromagnetic wave and photon? for more on this. With that out ...
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### Stimulated emission and No-cloning theorem

I think @garyp has given the correct answer, but it is worth expanding on it a bit. Stimulated emission is not cloning a state, but rather changes the state of a mode (characterized by wavelength, ...
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### What factor decides that when will scattering happen and when will reflection?

Reflection is a form of scattering, but it is coherent scattering. When scattering occurs from a lot of identical particles rigidly fixed in a plane, then all the possible ways a single photon can be ...
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### Full quantum model for spontaneous emission?

The minimal quantum model for spontaneous emission is given by the interaction Hamiltonian $$H = \int d\omega [g(\omega)a(\omega)\sigma^+ + g^*(\omega)a^\dagger(\omega)\sigma^-]$$ where \$a(\omega), a^...
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### If atoms both absorb and emit photons, then why are there still gaps in an absorption line spectrum?

Yes, materials can absorb high energy photons and release it as multiple lower energy photons. But also important is that the direction of the release is not the same as the absorption. If we have a ...
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### How do objects emit light?

An object that emits light would glow in the dark. If it only reflects light, it returns light that hits it.
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### Electrons in Atom in different energy states

When an atom absorbs a photon and changes to an excited state that photon is destroyed and no longer exists. It is not stored in the atom in some way. The photon is destroyed and its energy goes into ...
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### Why can Einstein coefficients be derived based on thermodynamically equilibrium relations when they are basically intrinsic?

Einstein's argument can be broken down into two parts. The first part involving stimulated emission concludes that in order for a charged two state system to be in thermal equilibria the external ...
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### If all matter can emit at all wavelengths, can all matter absorb at all wavelengths too?

The planck assumes a theoretical blackbody object that absorbs all incoming electromagnetic radiation and emits radiation at all wavelengths, depending on its temperature. The spectral distribution ...
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### How many cavity round trips typically occur inside laser resonators?

I'll try to address the direct question asked in the title of this question: "How many cavity round trips typically occur inside laser resonators?" The cavity finesse roughly captures the ...
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