excellent chart that shows the electromagnetic spectrum together with
typical uses of the various wavelengths. This really helps to
maintain perspective. Please take a few moments and look over the
chart.
Why terahertz and not mm-wave, micro, or radio waves? They all
penetrate matter easily. Radio waves penetrate too well so there is
not a good reflected signal plus the typical wavelengths are too long
to give any useful spatial resolution. Mm-waves do reflect a good
deal of the source radiation but the resolution is limited to about 1
mm. at best and typically a little less. Visible light doesn't
penetrate much at all. But terahertz has very good penetration (we're
speaking of non-polar and non-metallic materials) and significantly
better resolution than mm-waves. In addition, many molecules have
resonances in the terahertz frequency range which can be used to
identify the molecular species (spectroscopy).
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