What happens when an object starts to glow?

[Wille]

TL;DR

I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. But what is happening when an object starts to glow? I guess that the electrons of the vibrating atoms get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the vibrations of their molecules?

I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. Or possibly the other way around; the more vibrations, the more heat we say that the object has (right?). But what is happening when an object gets so hot that it starts to glow? I guess that the electrons of the vibrating molecules get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the large vibrations of their molecules?

Thanks!

 

[Drakkith]

There are many different ways to partition thermal energy into a material. If we model our material as a monoatomic gas, then the only way to do so is in their kinetic energy and any electronic transitions of the electrons. The more complexity we add to our model, the more ways we can add energy. Changing to a diatomic gas adds things like the vibration of each atom along their shared bond. For molecules that aren't diatomic, the vibrations take on upwards of six different forms: symmetric and asymmetric stretching, scissoring, rocking, wagging and twisting. See this video for an example of each one.

In addition, molecules can also rotate around certain bonds, and the more complicated the molecules get, the more ways all these different types of vibrations and rotations add together. The end result is a chaotic jumble of moving charged particles that are constantly bumping into and off of one another, getting pulled and twisted at random, and undergoing different transitions all the time. And don't forget that solid objects also generally share electrons, which are in motion throughout the bulk material and can collide with ions and other electrons, transferring energy to/from them. This is especially true for metals, which is why they are very good conductors of both heat and electric current.

The key idea here is that these particles undergo acceleration when they are subjected to pushing or pulling forces. What happens when charged particles undergo an acceleration? They radiate EM radiation! That's why hot objects glow. It's not solely because the electrons are undergoing electronic transitions. That's not even the dominant effect except for sparse gasses (like what you find in a neon light). The bulk of the light is the result of the acceleration of many charged particles in many different ways.

This also explains why hotter objects glow brighter and with a higher average frequency. The hotter the object is, the more energy each of these collisions have and the higher the acceleration, which results in a higher frequency of emitted light and more of it.

 

[Mentz114]

Summary: I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. But what is happening when an object starts to glow? I guess that the electrons of the vibrating atoms get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the vibrations of their molecules?

I understand it as heat is simply the motions of atoms/molecules (kinetic energy). The warmer an object gets (a solid), the more the molecules are vibrating. Or possibly the other way around; the more vibrations, the more heat we say that the object has (right?). But what is happening when an object gets so hot that it starts to glow? I guess that the electrons of the vibrating molecules get excited and then falls back and hence emitting photons (visible light). So why are the electrons excited? Why do they care about the large vibrations of their molecules?

Thanks!

A suffciently large cloud of matter can collapse under its self-gravity and begin nuclear fusion reactions - i.e. become a star.

 

[Wille]

There are many different ways to partition thermal energy into a material. If we model our material as a monoatomic gas, then the only way to do so is in their kinetic energy and any electronic transitions of the electrons. The more complexity we add to our model, the more ways we can add energy. Changing to a diatomic gas adds things like the vibration of each atom along their shared bond. For molecules that aren't diatomic, the vibrations take on upwards of six different forms: symmetric and asymmetric stretching, scissoring, rocking, wagging and twisting. See this video for an example of each one.

In addition, molecules can also rotate around certain bonds, and the more complicated the molecules get, the more ways all these different types of vibrations and rotations add together. The end result is a chaotic jumble of moving charged particles that are constantly bumping into and off of one another, getting pulled and twisted at random, and undergoing different transitions all the time. And don't forget that solid objects also generally share electrons, which are in motion throughout the bulk material and can collide with ions and other electrons, transferring energy to/from them. This is especially true for metals, which is why they are very good conductors of both heat and electric current.

The key idea here is that these particles undergo acceleration when they are subjected to pushing or pulling forces. What happens when charged particles undergo an acceleration? They radiate EM radiation! That's why hot objects glow. It's not solely because the electrons are undergoing electronic transitions. That's not even the dominant effect except for sparse gasses (like what you find in a neon light). The bulk of the light is the result of the acceleration of many charged particles in many different ways.

This also explains why hotter objects glow brighter and with a higher average frequency. The hotter the object is, the more energy each of these collisions have and the higher the acceleration, which results in a higher frequency of emitted light and more of it.

Thanks for this great answer. So it is the same for lamps then? It is not mainly electron transitions that make the metal thread emit light? But rather the accelerations of the atoms/molecules in the metal thread? And those accelerations I suppose happen due to the electrons trying to move through the thread when I turn on the switch to the lamp?

 

[Drakkith]

Thanks for this great answer. So it is the same for lamps then? It is not mainly electron transitions that make the metal thread emit light? But rather the accelerations of the atoms/molecules in the metal thread? And those accelerations I suppose happen due to the electrons trying to move through the thread when I turn on the switch to the lamp?

That's right. The electric current through the filament causes electrons to 'bump' into other electrons and into the ions making up the metallic lattice. This introduces vibrations into the lattice that, along with the collisions of the electrons, end up causing the entire filament to glow.

 

[Wille]

That's right. The electric current through the filament causes electrons to 'bump' into other electrons and into the ions making up the metallic lattice. This introduces vibrations into the lattice that, along with the collisions of the electrons, end up causing the entire filament to glow.

@Drakkith I have one final question in this matter. I thought I could continue this thread instead of writing a new one.
The lightning can heat the air to around 30 000 degrees C. The electrons move through the air due to the large potential between cloud and ground. Is it again the same reason why the kinetic energy (i.e. heat) of the air molecules increases as discussed above, i.e. that the electrons "bump in to" many air molecules as they pass to the next "available" atom? Or is it something else that gives the air the that enormous increase in kinetic energy? Since air is a gas I thought that maybe it should not be that crowed with atoms.

Thanks.

 

[PeterDonis]

what is happening when an object gets so hot that it starts to glow?

Nothing much different from before. See below.

I guess that the electrons of the vibrating molecules get excited and then falls back and hence emitting photons

Objects at any temperature above absolute zero are always emitting photons. The only difference is the average wavelength of the photons. Bodies at room temperature emit photons with an average wavelength in the infrared range, which is not visible to the human eye but can easily be detected with appropriate equipment. An object that is hot enough to visibly glow is simply emitting photons with an average wavelength in the visible light range.

It is true that there are multiple different mechanisms by which objects can emit photons (other posters have described some of them), and which mechanism is dominant can vary with temperature.

 

[hilbert2]

Modeling a solid as a 1D chain of atoms with Hookean bonds between them, the dispersion relation looks like

This is from http://lampx.tugraz.at/~hadley/ss1/phonons/1d/1dphonons.php

So the frequencies of the normal modes can't be higher than $\sqrt{\frac{4C}{m}}$, with $m$ the mass of an atom and $C$ the spring constant. So it would seem that you need vibrational transitions where the $n$ in $E = \hbar \omega \left( n+\frac{1}{2}\right)$ decreases by more than 1 to get a visible photon emitted.

 

[Drakkith]

The lightning can heat the air to around 30 000 degrees C. The electrons move through the air due to the large potential between cloud and ground. Is it again the same reason why the kinetic energy (i.e. heat) of the air molecules increases as discussed above, i.e. that the electrons "bump in to" many air molecules as they pass to the next "available" atom? Or is it something else that gives the air the that enormous increase in kinetic energy? Since air is a gas I thought that maybe it should not be that crowed with atoms.

I believe that's pretty much how it happens except for the fact that the electrons aren't really jumping from atom to atom. They get ripped from their atoms and fly freely through the empty space between the other electrons, ions, and air molecules until they collide with something, whereby they release lots of energy as radiation.

I'm sure the exact way that all this occurs is very complicated and requires an in-depth understanding of plasma physics.

 

[Dr_Nate]

Modeling a solid as a 1D chain of atoms with Hookean bonds between them, the dispersion relation looks like

View attachment 255636

This is from http://lampx.tugraz.at/~hadley/ss1/phonons/1d/1dphonons.php

So the frequencies of the normal modes can't be higher than $\sqrt{\frac{4C}{m}}$, with $m$ the mass of an atom and $C$ the spring constant. So it would seem that you need vibrational transitions where the $n$ in $E = \hbar \omega \left( n+\frac{1}{2}\right)$ decreases by more than 1 to get a visible photon emitted.

What you've shown here are acoustic phonons. It only applies to the charge from the nucleus. In real materials there will be optic phonons at higher energies.

But more importantly, in addition to the phonons, we must consider how the electrons can 'shake'. The relative permittivity $\tilde \epsilon(\omega)=\epsilon_1(\omega) + i\epsilon_2(\omega)$ determines how easily it is to shake (polarize) the electrons and lattice at each frequency.

From the relative permittivity $\tilde \epsilon$, the emissivity can be found as a function of frequency with some straightforward algebra. Multiplying the emissivity by the ideal blackbody radiation at some temperature will give you the actual observed spectrum of a real body at that temperature.

 

[hilbert2]

What you've shown here are acoustic phonons. It only applies to the charge from the nucleus. In real materials there will be optic phonons at higher energies.

Thanks. The usual way to derive Planck's law with statistical physics only tells the equilibrium amount of energy in modes of different wavelengths, but nothing about the mechanism how this equilibrium is reached..

 

[Dr_Nate]

Thanks. The usual way to derive Planck's law with statistical physics only tells the equilibrium amount of energy in modes of different wavelengths, but nothing about the mechanism how this equilibrium is reached..

Well then, just for completeness' sake, I'll mention that the electronic contribution to the relative permittivity arises from the electronic band structure and scattering mechanisms.