[LINK] Our wireless world...

[Stewart Fist]

Richard writes:

> "Wavelength in body tissue is not the same as in air"? - I am not quite
> sure what you mean - that the wavelength of the cellphone signal changes
> as the signal passes into the body? Yes, the body interacts differently
> with different frequencies, but a 1.8GHz passing through something is
> surely 1.8GHz on the other side...

Yes and No.  

Light passing through a lens  emerges at the previous speed, before it
entered the lens.  But while transiting the lens, it travelled at a lower
speed. (Ask Einstein how!)

So what I wrote is quite correct.

The wavelength is speed divided by frequency, and the speed of any EM wave
travelling through a solid object or the air, is less than it is in a
vacuum, therefore the wave-length changes in proportion.

This is exactly the same as our discussion a while ago about the speed of
light pulses in optical fibre.  The speed is inversely proportion to the
Refractive Index.  And for radio waves, bone and brains have different RIs
and hence produce different wavelengths within the bones and brain.

This only becomes important in these discussions because most electrical
engineers insist that the only way a radio signal can effect biological
tissue is if the signal resonates -- which implies that it is equal in
dimensions to the wavelength, or to a half wavelength, third, etc.  ie that
body tissue acts as a dipole.

However, a lot of the biological research suggests that interactivity is
possible with cell or organ dimensions well below a quarter wave-length.

DNA, for instance, is a relatively long-strand but wound into a very very
tiny coil.  It is also a semi-conductor, and this fact is exploited in the
comet-assay test to see if there are breaks in the strand.

-- 
Stewart Fist, writer, journalist, film-maker
70 Middle Harbour Road, LINDFIELD, 2070, NSW, Australia
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