absorbance

Examples of absorbance in the following topics:

• 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.

• 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.

• Greenhouse Gases and Global Warming

  • Greenhouse gases raise the Earth's equilibrium temperature by absorbing radiation that would otherwise be emitted into space.
  • The Earth absorbs this energy and then re-emits radiation in the infrared portion of the spectrum.  
  • The gases in the atmosphere, primarily CO2 and water vapor are highly absorbent in the infrared part 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 .

• 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.

• Sun Damage, Sunscreen, and Sunblock

  • Sunscreen (also commonly known as sunblock, sun tan lotion, sun screen, sun cream, or block out) is a lotion, spray, gel, or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation on the skin exposed to sunlight, and thus helps protect against sunburn .
  • Sunscreens contain one or more of the following ingredients: organic chemical compounds that absorb ultraviolet light; inorganic particulates that reflect, scatter, and absorb UV light (such as titanium dioxide, zinc oxide, or a combination of both); organic particulates that mostly absorb light-like organic chemical compounds but contain multiple chromophores, may reflect and scatter a fraction of light like inorganic particulates, and behave differently in formulations than organic chemical compounds.
  • Depending on the mode of action, sunscreens can be classified into physical sunscreens (i.e. those that reflect the sunlight) or chemical sunscreens (i.e. those that absorb the UV light).
  • The sunscreen on the left side of his face absorbs ultraviolet, making that side appear darker in the UV picture.

• UV-Visible Absorption Spectra

  • The resulting spectrum is presented as a graph of absorbance (A) versus wavelength, as in the isoprene spectrum shown below.
  • Since isoprene is colorless, it does not absorb in the visible part of the spectrum and this region is not displayed on the graph.
  • Because the absorbance of a sample will be proportional to the number of absorbing molecules in the spectrometer light beam (e.g. their molar concentration in the sample tube), it is necessary to correct the absorbance value for this and other operational factors if the spectra of different compounds are to be compared in a meaningful way.
  • (where A= absorbance, c = sample concentration in moles/liter & l = length of light path through the sample in cm. )
  • Such light absorbing groups are referred to as chromophores.

• Water’s High Heat Capacity

  • Water is able to absorb a high amount of heat before increasing in temperature, allowing humans to maintain body temperature.
  • The capability for a molecule to absorb heat energy is called heat capacity, which can be calculated by the equation shown in the figure .
  • When heat is absorbed, hydrogen bonds are broken and water molecules can move freely.
  • Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius.