Radiation, Absorption

RADIATION IS ENERGY transmitted by electromagnetic waves. Electromagnetic waves travel at the speed of light (when passing through a vacuum) and have a characteristic wavelength, λ, which is inversely proportional to their frequency, v, by

where c is the speed of light. Electromagnetic radiation is conceptualized in contemporary theory both as a wave and as a stream of particles called photons (this dual approach is referred to as wave-particle duality). The energy of any photon, E, of radiation is inversely proportional to the wavelength by

where h is Planck's constant. This relationship allows us to order electromagnetic waves from high energy/short wavelength (for example, x-rays), to low energy/long wavelength (for example, radio waves). The resulting progression is referred to as the electromagnetic spectrum (Figure 1). The visible region of the electromagnetic spectrum is bound by infrared (IR) radiation on the lower energy side of the visible region (around 1μm to 1 mm in wavelength), and by UV radiation (UV) on the higher energy side (from 400 nm to 1 nm). Microwave radiation is slightly lower in energy than IR, with a wavelength of around 1 cm.

Figure 1: The electromagnetic spectrum

All objects both emit and absorb radiation. Although all objects emit radiation at all wavelengths, the frequency of maximum emission, Xmax, is proportional to the temperature of the object by Wiens law

where α is a constant equal to 2897 μm K. This implies that hotter objects emit higher energy radiation, as would be expected from everyday experiences. From Wiens law, the surface temperature of the sun can [Page 838]be calculated based on its emission peak at ∼0.5 μm (green light) to be around 5800 K. The average temperature of the Earth's surface is around 18 degrees C (290 K) which corresponds to a peak emission at around 10 μm, in the infrared to microwave region.

There are three basic modes of motion: translational (movement through space), rotational, and vibrational. These are important, because along with electronic energy, they are the ways in which gas molecules can store energy. Quantum theory dictates that energy levels are discrete, not continuous; this implies that molecules will only absorb discrete frequencies of radiation that correspond to the gap between a high and lower energy state. UV radiation corresponds to the gap in energy between electronic energy levels in a molecule. When a molecule absorbs UV radiation, it may be promoted to an electronically excited state. In the general, this will make the bonds holding the atoms together weaker and may help facilitate reactions or the breakup of molecules. For example, the reactions that complete the Chapman mechanism in the stratosphere:

The Chapman mechanism is chemically a null cycle, but it is important in the context of life, as it prevents most of the high-energy radiation from below 320 nm reaching the Earth's surface. IR and microwave radiation, being lower in energy than UV, correspond to the gaps between rotational and vibrational energy levels, respectively. Quantum theory dictates that molecules interact with IR/microwave radiation only when two conditions (selection rules) are

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