Infrared as Heat Waves: Why Only IR?

[Pranav Jha]

I was wondering why only infrared waves are known as heat waves. Electromagnetic waves of all frequencies carry energy and when absorbed by electrons increase the kinetic energy of the electron absorbing the wave. So,the average kinetic energy of the atom of the electrons (thereby temperature) of the atom must increase. So, shouldn't all the waves be called heat waves?

 

[Pranav Jha]

On reading a book i got the following answer:
Infrared waves, with frequencies lower than those of visible light, vibrate
not only the electrons, but entire atoms or molecules in the structure of the
glass. This vibration increases the internal energy and temperature of the struc-
ture, which is why infrared waves are often called heat waves.

What does vibrating an entire atom mean? and why does vibrating electron alone not increase the temperature?

 

[drummin]

I always thought they are called heat waves because the black body radiation from hot household objects such as a pan on a stove peak at the infrared wavelengths. "Heat waves" is an archaic term from kitchen experiences.

 

[Pranav Jha]

so, are they called heat waves primarily because they comprise the majority of the radiation?

 

[alxm]

Well, at the temperatures we're used to, it's mostly rotational-vibrational transitions in molecules, which means the kinetic motion of the nuclei of the atoms in the molecules, rather than the electrons. The nuclei are very heavy and move very slowly compared to the electrons, so these states have lower kinetic energy.

Higher wavelengths don't heat things because they're much higher in energy than equillibrium. An electron will get excited, and then return to its ground state giving off the same energy, or nearly the same energy. Only a small amount turns into rotational/vibrational energy and heats it up.

To make an analogy: Think of a bunch of balls shaking around in a bucket. The shaking motion is your temperature. Then you throw a ball into the bucket with a lot of force. It will bounce out. Or it will hit another ball and the other ball will bounce out. Not much will be transferred to the balls that are in the bucket, because it was so much higher in energy, its motion was largely decoupled (as one says) from their motion. But if you throw in a ball more softly, it'll stay in and all its energy will join the equilibrium.

Anyway, your original point is still correct. There isn't any particular region of the spectrum (or any particular form of motion/energy) that corresponds to 'heat'. The thermal energy gets distributed between all the various degrees of freedom that the system has. IR and rotational/vibrational transitions only corresponds to heat at the temperatures we're used to. We all know how things glow if they get hot enough, and it's in the visual-UV range that you start getting electronic transitions, and supposedly, black-hole accretion discs can get so how that their thermal spectrum is in the x-ray range!

 

[Pranav Jha]

Well, at the temperatures we're used to, it's mostly rotational-vibrational transitions in molecules, which means the kinetic motion of the nuclei of the atoms in the molecules, rather than the electrons. The nuclei are very heavy and move very slowly compared to the electrons, so these states have lower kinetic energy.

Higher wavelengths don't heat things because they're much higher in energy than equillibrium. An electron will get excited, and then return to its ground state giving off the same energy, or nearly the same energy. Only a small amount turns into rotational/vibrational energy and heats it up.



Let's consider a free electron in a metal, which is not bound to a specific atom and thus (i guess) doesn't have to stay at a fixed energy level. So, considerting that can a metal be heated by shining a high frequency wave under normal conditions as the electron doesn't have to fall to a fixed energy level and thus all the absorbed energy can be transferred into the vibrational and rotatory kinetic energy ( presuming there is no photoelectric emission)?

 

[sophiecentaur]

It's only a matter of how they've been classified and it is consequently, a bit sloppy. I think it's largely based on the sort of sources from which waves in this particular frequency band are produced more than other frquencies.- i.e. hot objects

You might also ask why em radiations of the same frequency can be called gamma rays or x rays. In that case, it has got to be how they're produced as they are identical in all other respects.

This sort of question reflects an unhelpful but possibly unavoidable system which gives young students over-simplified 'classification' for substances and phenomena. Take solids, liquids and gases. There are many substances that fall into more than one or none of those classifications but we grow up assuming that everything can be classified in this way.