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Name: Shoaib W.
Status: student
Age: 17
Location: N/A
Country: N/A
Date: 2000-2001

as the frequency of electromagnetic wave increases their penetration power decreases ie from microwavew to infrared to light waves the penetration power decreases. But X rays and gamma rays which have a frequency greater than that of light have high penetration power. Is this an anomalous behaviour? why does the penetration power decrease with increase in frequency?

You are starting with a false premise. The penetrating power of EM radiation does NOT necessarily decrease with increasing frequency. The penetrating power THROUGH WHAT MEDIUM needs to be specified. Microwaves cannot escape from your microwave oven because the metal, and that wire screen on the glass door prevent them from escaping, i.e. they have no penetrating power. There are many coatings that are applied to glass that makes the glass a good reflector of infrared radiation. Carbon dioxide and water for example are also strong absorbers of certain wavelengths of infrared radiation. X-rays easily penetrate metals that have low atomic number, like aluminum, but are strongly absorbed by metals that have a high atomic number, like lead.

The determining factors are of course the frequency or wave length of the EM radiation, but the ability of the medium through which the radiation is moving is equaly important.

Vince Calder

As the frequency increases from radio waves to microwaves to IR & visible light the energy of the photons matches rotational, vibrational, and electronic energy level transitions of typical molecules. As a result, the energy is absorbed and penetration depth is small. For very high energy photons, such as deep UV, x-ray, gamma ray, etc. the energy of the photons is so large that it does not match any electronic energy transitions in matter and so the photons are not absorbed but penetrate deeply.

Greg Bradburn

There are many mechanisms by which materials absorb electromagnetic energy: molecules have massive parts that can jiggle at frequencies mostly in the infrared range of the spectrum; most solids (particularly crystals) have collective oscillations of many atoms, at frequencies corresponding to low-energy x rays; individual atoms rearrange or eject electrons, and this takes energies corresponding to frequencies from around those of visible light to those of hard x rays.

We evolved to use visible light for seeing things because this range of frequencies penetrates well through air and also tends to carry useful information about solid materials it has reflected from, or been absorbed and re-emitted by.

Tim Mooney

There is nothing specific about a frequency of electromagnetic radiation that makes it more or less penetrating. The key is its interaction with matter, specifically if the photon's energy is right to excite some transition of a charged particle.

For instance, microwaves penetrate glass very easily, but they are strongly absorbed by water. Move up to slightly higher frequency, and infrared is strongly absorbed by both glass and water. Both substances transmit visible light. Ultraviolet is stopped by glass, but not so readily by water. So it's not a simple correlation between transmittance and wavelength.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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