Electromagnetic Radiation, Black Bodies and Matter

There are broadly seven ways in which a substance interacts with the electromagnetic spectrum: – Absorption, Reflection, Refraction, Diffraction, Photoelectric Effect, Scattering, and Emission or Radiation. There are only 2 types of electromagnetic radiations that matter can emit:-

  1. Due to a chemical reaction (includes nuclear radiation)
  2. Due to accelerating charges

Radiation due to changing current (accelerating charges wrt. their current position or speed)

If you change the flow of current fast enough, it can produce electromagnetic radiation. On a more molecular level, if you suddenly change the speed of an electron, it will produce an electromagnetic wave. Conversely, if you pass an electron through an EM wave, it will suddenly change speed (or produce a current). This is, by the way, how TV antennas work. Another very simple example of this is how we produce radio waves.

Radiation due to a chemical reaction

In a candle, the chemical reaction between the wax vapour and oxygen is exothermic, and releases energy in the form of heat and light. Both heat and light are released as electromagnetic waves, waves released in the visible region are called light and waves released in the infrared region are called heat. Similarly, a nuclear chemical reaction will release nuclear radiation. It is the same concept as a candle, only the radiation released is very powerful, called ionizing radiation, which as the name suggests, can create ions out of stable compounds.

Radiation due to Temperature – Thermal Radiation

Almost all radiation can be attributed to accelerating charges. Is radiation in chemical reactions also due to accelerating charges? We’re not sure. But we are sure that Thermal Radiation is. What’s that?

An iron nail when heated to 3000 K glows. Actually it glows at all temperatures, it’s just that we can see the glow when it reaches a certain temperature. Matter exhibits this property, that at every possible temperature above 0 on the Kelvin scale, it glows. All types of matter do this, and at all temperatures. The propane gas that is heating the nickel itself glows, but its glow is hidden behind the light radiation from the chemical reaction. (Reference) Which brings us to the second point: –
The glow from temperature is much fainter than a glow from chemical reactions. 

At the molecular level, radiation translates to movement (or as we have just seen, acceleration of charges). However, this movement is not to be confused with electron movement in orbitals or protons rotating about their own axes. This is the movement of the atom or the molecule as a whole body. At the molecular/atomic level, heat translates into kinetic energy possessed by the atoms/molecules. As atoms and molecules move, the charges within them also move w.r.t their stationary positions. Don’t get me wrong though, electrons are never stationary (I’m not 100% sure as I can’t see an electron), but from the frame of reference of someone looking at the atom from the outside, seeing it dance about, they are also moving in the motion of that atom. And this and only this motion’s acceleration gives thermal radiation.

Vibrational Quantum States

The only region in the spectrum that associates with heat is the IR region. Turns out, a photon from the infrared region has an energy on the order of the energy of vibrational transitions in molecules. The reason why IR light is produced and associated with heat is that you are seeing molecules go from one vibrational quantum state to a lower vibrational quantum state by giving off a photon of appropriate energy (in the IR region). (Reference) Are these related to phonons or rotons? Tell us in the comments.

When an object is above 0 K, it radiates infrared waves. As the temperature crosses 1000 K, the radiations become visible as their wavelengths shorten, while intensity increases several fold. Higher the temperature, higher is its vibrational state, higher the energy radiated.

For an object with a temperature T (in Kelvin) and a surface area A, the energy radiated (Q) in a time t is given by the Stefan-Boltzmann law of radiation:

The constant e is known as the emissivity, and it’s a measure of the fraction of incident radiation energy is absorbed and radiated by the object. This depends to a large extent on how shiny it is. If an object reflects a lot of energy, it will absorb (and radiate) very little; if it reflects very little energy, it will absorb and radiate quite efficiently. Black objects, for example, generally absorb radiation very well, and would have emissivities close to 1. This is the largest possible value for the emissivity, and an object with e = 1 is called a perfect blackbody. (Reference)

Black Bodies

Blackbodies are assumed to have emissivity as unity (e=1) which means they emit everything back after absorption. Another assumption is that they are perfect absorbers so they absorb all the wavelengths and reflect none, which is why at 0K they will be completely black. Blackbodies are perfect world scenarios for thermal radiation. Using blackbodies as a standard helps to compare radiation spectra of various other materials.

A closed box with walls of graphite at a constant temperature with a small hole on one side produces a good approximation to ideal black-body radiation emanating from the opening. (Source)

Difference between Microwaves and Ovens

Both ovens and microwaves use electromagnetic waves to heat our food, the difference is in the type of EM waves they use.

A simple oven’s working is simple too. An oven heats the air inside the vessel, increasing the temperature of and around all of your food. Your food absorbs the heat (i.e. IR radiation) from the air around and inside it, and in turn itself starts to radiate infrared waves (i.e. becomes hot) and is cooked.

A microwave oven uses radio waves with an average wavelength of a few centimeters. Radio waves, unlike visible light, are not absorbed or reflected by the food. The only thing they do is create a rapidly alternating electric and magnetic field around our food molecules. Water acts as a dipole and loses energy in the form of heat. This is because of dielectric loss, and works especially well on water because it has a high dielectric constant. The exact frequency used is slightly away from the frequency at which maximum dielectric loss occurs in water to ensure that the microwaves are not all absorbed by the first layer of water they encounter, therefore allowing more even heating of the food. (Reference)

What is Dielectric Loss? Imagine stirring a cup clockwise and then suddenly stir it anti-clockwise, and then clockwise again and so on. The energy lost in switching the polarity is called dielectric loss. (Reference)

But if E=hv, and visible light has higher frequency than microwaves, why don’t we use visible light to heat up food?

  1. Energy isn’t just dependent on frequency, it also depends on intensity.
  2. Visible light doesn’t correspond to heat energy released by matter.
  3. Even if we do use visible light, the upper most layer of our food will interact the most with it, leaving the inside cold.

(Reference)

How TV signals work

The light (i.e. radio frequency) causes electrons in the TV aerial to oscillate and this oscillation generates an oscillating electric current. The voltage this generates is amplified by your TV. At the TV transmitter the same happens in reverse: an oscillating voltage is applied to the TV transmitter, the electrons oscillate in response and the oscillation generates an electromagnetic wave. (Reference) (EM waves can only be generated via atoms only)

A microwave works in a similar way, the only difference being that in the microwave engineers try to maximize dielectric heat loss, while in a T.V. signal receiver, they’ll try to minimize it.

Does everything absorb and emit infrared all the time?

Different objects have different absorption rates for different lights. Most plastics allow IR to pass through. Glass will block low frequency IR (red hot), but allow the passage of high frequency (white hot) IR. Hence, the heat of the sun will easily pass into a greenhouse, but once this energy is converted into low frequency heat by the objects within that absorb it, then the resulting low frequency heat is trapped. Hence, the Greenhouse Effect. (Reference)

You should also read: –

How NMRI Works
How X-Ray Works

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