How would you distinguish between thermal and nonthermal radiation?

Electromagnetic waves are fundamental to understanding the nature of thermal and nonthermal radiation. They are ripples of energy caused by the acceleration of charged particles, which spread out through space and can travel through a vacuum. These waves include a broad spectrum of types, from radio waves with long wavelengths to gamma rays with very short wavelengths.

In the context of radiation, electromagnetic waves carry energy away from various sources. The energy emitted by these waves varies across the electromagnetic spectrum, with waves like X-rays carrying a lot more energy than, for example, radio waves. When looking at radiation given off by objects due to their temperature, such as the heat we feel from the sun, we're referring to a subsection of this spectrum—primarily infrared radiation but also visible light and ultraviolet radiation. On the other hand, nonthermal sources like radio antennas emit electromagnetic waves without the involvement of high temperatures.

To delve deeper, it's essential to understand Planck's law. This principle is concerned with the radiation emitted by a black body in thermal equilibrium at a fixed temperature, and it is fundamental for studying thermal radiation. A black body is an idealized object that absorbs all radiation that hits it, without reflecting any, and re-emits radiation at a spectrum that depends solely on its temperature.

Planck's law describes the intensity of this radiation as a function of wavelength and temperature. The key result from Planck's law is that the radiation emitted by a body gets spread over a continuum of wavelengths, and the distribution of energy among these wavelengths changes with temperature. For example, as an object gets hotter, it not only emits more radiation, but the peak of the emitted spectrum shifts towards shorter wavelengths, which is why very hot objects can begin to glow with visible light.

The concept of absolute zero is intrinsically tied to the nature of thermal motion in particles. Absolute zero, defined as 0 Kelvin or -273.15 degrees Celsius, is theoretically the lowest possible temperature where all classical motion of particles within a substance ceases.

The relevance to radiation is that at any temperature above absolute zero, substances have some amount of thermal motion, and it's this motion that causes the emission of thermal radiation. The higher the temperature, the more vigorous the motion of the particles and thus the greater the energy and intensity of the emitted thermal radiation. In contrast, nonthermal radiation doesn't necessarily require particle motion related to heat and can occur at or near absolute zero, as it depends on other mechanisms such as electronic transitions or magnetic field changes.

Charged particles play a crucial role in both thermal and nonthermal radiation. In thermal radiation, the movement and vibration of charged particles within atoms and molecules at a given temperature result in the emission of radiation. This is because any accelerated charge will emit electromagnetic waves.

In nonthermal radiation, charged particles are also involved but in different processes. For instance, when electrons transition between energy levels in an atom, they emit photons—packets of light not directly related to the atom’s thermal state. Another example includes charged particles being accelerated by magnetic fields in space, emitting radiation in the form of radio waves or even visible light. Here, the acceleration is not due to heat but rather to other energy changes happening within a system.