P3D Re: projection

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Thu, 25 Feb 1999 20:09:51 -0700
>From: jacob@xxxxxxxxxxxxxx (Gabriel Jacob)
>Subject: P3D Re: projection

>As to the reference about cameras using IR to produce an image that
>we can see, the camera is converting an invisible portion of the
>spectrum into something we can see (i.e. light). 

If we're going to be really correct, and make such a distinction between
"radiating heat" and "radiating infrared energy", then please explain
just how it is that a film camera 'produces light that we can see'. :-)

>This doesn't make
>the IR portion light. As to animals, it's true they can see some
>IR and UV which is invisible to humans but again, this doesn't
>redefine that portion of the spectrum to "light". 

Never mind the animals - what about humans? Some people who have had
cataract surgery can see much shorter wavelengths than the general
population. Does that redefine light for all humans, or just for them?
What if the entire human population has such surgery - does that redefine
light? What if only 49 percent of the population has the surgery, so
"the majority" of the population still can't see the shorter wavelengths?

What if it's in the frequency range of visible light, but it's too dim for
you to see it - is it still light? As your eyes get dark-adapted, does the
definition of light change?

So far, I like John Toeppen's definition better than all the others posted -
he's the only one who sounds like he might have looked in an actual physics
text to find the contemporary usage. I don't have access to such a text
at present, but I suspect the division he described (microwave, light, X-ray)
is based on *major* differences in the interaction of the EM energy with
matter.

>As for equations applying to UV and IR as well as the visible 
>spectrum this is correct, but since they have different physical
>effects on matter (or 3-D slides!) the classifications (IR, light,
>UV, etc.) are important to keep distinct.

But do those different physical effects (relatively minor compared with
X-ray/microwave differences) have wavelength boundaries that necessarily
line right up with the threshold of sensitivity of the human eye? First
you want to distinguish light from non-light by human perception, then
you want to *also* apply criteria of physical interaction with matter.

I'm much more concerned with the definition of "infrared" than with the
definition of "light". The range covered by the term "infrared" includes
wavelengths with very different thermal properties from one another, and
that are predominantly produced under very different circumstances. 
The confusion between near IR and far IR seems to cause much more trouble
than confusion regarding the definition of "light".

This whole business of defining light in terms of what most of us can
see seems to have so many difficulties, I think it may be clearer to
call what most of us can see "visible light", and the other kinds
"infrared light" and "ultraviolet light", and for the generic term
"light", stick with a physics definition.

"If a tree falls in the forest, and nobody was there to hear it, then
where were they?"

John R


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