selective absorber

Examples of selective absorber in the following topics:

• Greenhouse Gases and Global Warming

  • The extra temperature comes from the Earth and it's atmosphere selectively absorbing certain wavelengths of radiation while reflecting other wavelengths.
  • The gases of the atmosphere are "selective absorbers"; energy in the visible part of the electromagnetic spectrum passes through the atmosphere directly to the Earth's surface (with some reflection occurring as well).
  • The Earth absorbs this energy and then re-emits radiation in the infrared portion of the spectrum.  
  • The atmosphere absorbs the infrared radiation from the Earth, preventing it from escaping to space.  
  • Radiative transfer dictates what energy is reflected, absorbed, and emitted .

• Microwaves

  • The thermal motion of atoms and molecules in any object at a temperature above absolute zero causes them to emit and absorb radiation.
  • The range of microwave frequencies is specially selected so that the polar molecules, in trying to maintain their orientation with the electric field, absorb these energies and increase their temperatures—a process called dielectric heating.

• The Photoelectric Effect

  • In the photoemission process, if an electron within some material absorbs the energy of one photon and acquires more energy than the work function of the material (the electron binding energy), it is ejected.
  • Electrons can absorb energy from photons when irradiated, but they usually follow an all-or-nothing principle.
  • If excess photon energy is absorbed, some of the energy liberates the electron from the atom and the rest contributes to the electron's kinetic energy as a free particle.
  • The photocathode contains combinations of materials, such as caesium, rubidium, and antimony, specially selected to provide a low work function, so when illuminated by even very low levels of light, the photocathode readily releases electrons.

• Fluorescence and Phosphorescence

  • Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
  • In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation.
  • However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation.
  • The emitted radiation may also be of the same wavelength as the absorbed radiation, termed "resonance fluorescence".
  • Unlike fluorescence, a phosphorescent material does not immediately re-emit the radiation it absorbs.

• Absorption of Light

  • Light energy initiates the process of photosynthesis when pigments absorb the light.
  • Organic pigments have a narrow range of energy levels that they can absorb.
  • Chlorophyll a absorbs light in the blue-violet region, while chlorophyll b absorbs red-blue light.
  • Plants on the rainforest floor must be able to absorb any light that comes through because the taller trees absorb most of the sunlight and scatter the remaining solar radiation
  • Each pigment has (d) a unique absorbance spectrum.

• Vibrational Spectroscopy

  • Furthermore, the number of observed absorptions may be decreased by molecular symmetry, spectrometer limitations, and spectroscopic selection rules.
  • One selection rule that influences the intensity of infrared absorptions, is that a change in dipole moment should occur for a vibration to absorb infrared energy.

• Absorption

  • You can't absorb radiation that isn't there.
  • Phenomenologically you can imagine that there are many independent absorbers in the beam, each with a cross section $\sigma_\nu$ and a number density $n$.
  • You can think of this as the cross section per unit mass of the absorbers.

• The Greenhouse Effect

  • The greenhouse effect is an elevation in surface temperatures due to atmospheric gases absorbing and re-radiating thermal energy.
  • This thermal radiation from the surface has a much longer wavelength than the solar radiation that was initially absorbed.
  • The majority of gases in the atmosphere, such as nitrogen, oxygen, and argon, cannot absorb this infrared radiation.
  • Gases known as greenhouse gases, including water vapor, carbon dioxide, ozone, and methane, absorb and trap this heat as it tries to escape from the atmosphere.
  • The cloud layer can also absorb infrared radiation and contribute further to the greenhouse effect.

• Absorption in the Small Intestine

  • Examples of nutrients absorbed by the small intestine include carbohydrates, lipids, proteins, iron, vitamins, and water.
  • The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances such as the proteins required by our body.
  • Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions: iron is absorbed in the duodenum; vitamin B12 and bile salts are absorbed in the terminal ileum; water and lipids are absorbed by passive diffusion throughout the small intestine; sodium bicarbonate is absorbed by active transport and glucose and amino acid co-transport; and fructose is absorbed by facilitated diffusion.

• Dosimetry

  • Radiation dosimetry is the measurement and calculation of the absorbed dose resulting from the exposure to ionizing radiation.
  • Non-SI units are still prevalent as well: absorbed dose is often reported in rads and dose equivalent in rems.
  • The distinction between absorbed dose (Gy/rad) and dose equivalent (Sv/rem) is based upon the biological effects.
  • That is to say, for the same absorbed dose in Gy, alpha particles are 20 times as biologically potent as x-rays or gamma rays.
  • Dose is a measure of deposited dose and therefore can never decrease: removal of a radioactive source can reduce only the rate of increase of absorbed dose -- never the total absorbed dose.