P3D Re: light, stricly speaking....

[roberts@xxxxxxxxxxxxxxxxx (John W Roberts)]

>Date: Fri, 26 Feb 1999 20:23:26 -0700
>From: jacob@xxxxxxxxxxxxxx (Gabriel Jacob)
>Subject: P3D Re: light, stricly speaking....

>>Light is the form of electromagnetic radiation that stretches from Xrays
>>to microwaves (~5nm to ~20 microns in length).  Visible light is a much
>>narrower range, from 400nm to 750nm.

>Using the same logic, one can say...
>HEAT is the form of electromagnetic radiation that stretches from X-rays
>to microwaves (~5nm to ~20 microns in length). Visible HEAT is a much
>narrower range, from 400nm to 750nm. 

Heat is not *any* form of electromagnetic radiation at all! :-)
The analogy is not useful.

>Seriously folks, I think this shows how faulty the logic is, that light
>can happen to be visible and invisible. I have to agree that over the
>years the term light has been broadened to include other portions of
>the spectrum but strict usage of the word light is still referred to
>ONLY the visible portion of the spectrum. 

I actually went to a technical library to check this out, and looked in
a goodly number of respected publications for the definition of "light".
I found three definitions, typically at least two per publication:
 1) the visible range of electromagnetic radiation, roughly 400nm to 700nm.
 2) the range of electromagnetic radiation including infrared, visible,
      and ultraviolet.
 3) the entire electromagnetic spectrum.

There was not any note that any of these definitions was more valid than
the others. This is the accepted usage.

Earlier in the debate on the definition of "light", one poster criticized
another for "redundancy" in the use of the term "visible light". All of the
technical publications I checked used the term "visible light" without shame
or apology, so I do not believe that earlier criticism was merited.

I would speculate that the common accepted use of the term "light" to
apply to ultraviolet and IR may have much of its basis in history - these
ranges were first detected by the use of optical equipment, and were
perceived as continuations of the optical spectrum. It also makes some sense
to continue the use of this terminology - UV and IR are generated and
manipulated using techniques that are much closer to those for visible light
than those for X-rays and microwaves. And transmissivity for a wide range of
common objects (for example, a piece of black poster paper) is fairly
consistent across much of the UV-IR range, and different from that in the
microwave and X-ray ranges.

>It is convenient in some
>disciplines to describe heat, light, sound, etc. loosely, because
>depending on the discipline, the specific mechanical or electromagnetic
>characteristic of interest might span across the traditional defined
>classifications. However this doesn't make the loose terminology
>correct. 

It is also more incumbent on a person wishing to make a correction to
be careful not to use loose terminology in making the correction (e.g.
the definition of heat, and the description of what a camera does).

John R


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