Name: Shoaib W.
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.
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.
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.
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
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.
PG Research Foundation, Darien, Illinois
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Update: June 2012