gamma-rays - - - - - - - -highest energy
---------------- 1 A
X-rays
---------------- 10 A
"soft" X-rays
---------------- 100 A
UV
---------------- 400 nm
visible light
---------------- 800 nm
near - IR
---------------- 2.5 um
mid - IR
---------------- 25 um
far IR
---------------- 400 um
microwaves
---------------- 25 cm
radiowaves - - - - - - - - -lowest energy
Molecules are made of atoms 'chemically' bonded togther - the atoms share electrons. Molecules also have various modes of vibration, including translation, rotation, and vibration of the atomic bonds. Each is excited at a different frequency. Furthmore, assymmetric molecules like water have dipole moments.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/diph2o.html
http://content.aip.org/JCPSA6/v59/i5/2254_1.html
and Google on "dipole moment of water".
Tissue is generally transparent to radiowaves which generally have low energy. The longest waves pass right through with no interaction.
The vibration modes of water like those of other molecules can be stimulated primarily by microwaves (as in microwave oven or radar).
"Infrared radiation is absorbed and emitted by the rotations and vibrations of chemically bonded atoms or groups of atoms ( and thus by many kinds of materials)", and is generated by atomic vibrations which can be induced by electrical resistance (as in heating elements). See next ref:
http://zebu.uoregon.edu/~js/glossary/infrared.html
http://www.boselec.com/products/irchosou.shtml
www.spectroscopyeurope.com/IR_15_6.pdf[/URL] (10-100 microns)
Skin can absorb infrared to some extent - one can feel the warmth or heat. Too much IR can burn tissue.
Light interacts by photoelectric effect (which has a threshold) or molecular interaction. Human skin is adapted to visible light, and is somewhat transparent to light. The eyes are specially adapted to visible light. Light can be concentrated (as with a convex prism) or focussed such that the energy flux could heat or burn a surface (like skin).
UV, just beyond the visible spectrum penetrates the skin and can damage cells.
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ref: http://www.westonaprice.org/nutrition_guidelines/nutrition_vitaminD.html
Ultraviolet (UV) light is divided into 3 bands or wavelength ranges, which are referred to as UV-C, UV-B and UV-A.6 UV-C is the most energetic and shortest of the UV bands. It will burn human skin rapidly in extremely small doses. Fortunately, it is completely absorbed by the ozone layer. However, UV-C is present in some lights. For this reason, fluorescent and halogen and other specialty lights may contribute to skin cancer.
UV-A, known as the "tanning ray," is primarily responsible for darkening the pigment in our skin. Most tanning bulbs have a high UV-A output, with a small percentage of UV-B. UV-A is less energetic than UV-B, so exposure to UV-A will not result in a burn, unless the skin is photosensitive or excessive doses are used. UV-A penetrates more deeply into the skin than UV-B, due to its longer wavelength. Until recently, UV-A was not blocked by sunscreens. It is now considered to be a major contributor to the high incidence of non-melanoma skin cancers.7 Seventy-eight percent of UV-A penetrates glass so windows do not offer protection.
The ultraviolet wavelength that stimulates our bodies to produce vitamin D is UV-B. It is sometimes called the "burning ray" because it is the primary cause of sunburn (erythema). However, UV-B initiates beneficial responses, stimulating the production of vitamin D that the body uses in many important processes. Although UV-B causes sunburn, it also causes special skin cells called melanocytes to produce melanin, which is protective. UV-B also stimulates the production of Melanocyte Stimulating Hormone (MSH), an important hormone in weight loss and energy production.
UVA 400 nm - 320 nm
UVB 320 nm - 290 nm
UVC 290 nm - 100 nm
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X-rays (Roentgen rays, or Roentgenstrahlung) lie beyond UV. X-rays are generated normally by bombarding heavy metals with electrons accelerated to several KeV. The electrons 'knock out' an electron preferably in the K-shell (closest to nucleus) or L-shell. An electron falling into this shell will emit a UV photons. Anyone who has had an X-ray knows first hand of the penetrating power of X-rays. The shorter the wave length (higher frequency) the more penertrating the X-ray.
Gamma-rays originate from decay of the radioactive nuclei and some subatomic particls. They are the most penetrating of radiation.
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Gamma rays and X-rays can cause chemical reactions with the cells, in a process called "radiolysis". This means breaking atomic bonds within molecules (DNA, RNA, proteins, etc), thus damaging or destroying the vital chemicals within the cell. In addition, power X-rays and gamma-rays may deflect electrons from atoms (Compton effect) and the electrons ionize the water and compounds inside the cells. With all of the water present in cells, one common product from radiolysis is hydrogen peroxide (H2O2), which is a powerful oxidizer. H2O2 can oxidize and destroy important compounds in the cell - a good reason to take antioxidants.
