Re: Visibility and wavelength

[Paul Anderson <hacker@geeky1.ebtech.net>]

On Mon, 5 Jul 1999, Jonathan Shawl wrote:

> This is a dark gray area to me, I hope you can shed some "light" on it for me.
> :-)
>
Long wavelength == IR
Short Wavelength == Ultraviolet

Those are the two ends of the light spectrum, and frequency is inversely
proportional(IIRC) to wavelength, so the lower the frequency, the larger
the wavelength.  If you're talking about millimeter wavelengths, then
you're talking stuff like 2.4GHz radio, and at those frequencies water
sucks it up like lead plate.  That's why radar doesn't work under water(in
fact, that's also why nuclear fuel rods are stored in deuterium oxide,
heavy water).  That's also why the sky is blue, and the ocean is blue.
With the sky, longer wavelengths than blue are blocked by the ozone
layer(guess what causes sun burns(which are in fact radiation burns) and
is the principle agent of heat?  yes!  IR!).  In the ocean, all other
colours are obsorbed by the water, and only the blue can get back to your
eyes because of it's shorter wavelength.

Which is also the fundamental concept behind DVD's.  Prior to DVD
technology, it was very difficult to produce a blue laser, therefore an
infrared laser is used in CD-ROM drives for reading the disc.  The longer
wavelength demanded larger 'notches' in the disc, thereby reducing the
storage capacity of the disc.  Then, a couple years ago a method was
discovered to cheaply and effectively make blue lasers, with the shorter
wavelength of blue light you can focus on much smaller 'notches' on the
surface of the disc, while still being able to read a CD-ROM drive.  TTYL!


---
Paul Anderson
hacker@geeky1.ebtech.net
"This is Maurice, our sheep-herding carrot."
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