RE: Infrared & Water

[Stan McQueen <stan@xxxxxxxxxxx>]

At 07:52 PM 03/20/2000 -0500, you wrote:
>"...and as anyone who has taken a photo across a body of water will 
>attest, water absorbs the wavelengths of IR that IR film records and thus 
>looks dark!"
>
>Just took a look at a glass of water with an IR camcorder.  Looks, crystal 
>clear to me, just like it does under regular visible light.  The main 
>reason water looks dark in many IR photos is because it is reflecting the 
>sky, NOT because it is a good absorber of IR light.  Before suggesting 
>explanations to a phenomenom it would be beneficial to test!  Peace Rolland

Actually, Rolland, your test has little or nothing to do with the question, 
since the question refers to whether water absorbs IR from light being 
reflected from it. Your test would answer the question of whether water 
absorbs IR from light being transmitted through it. And of course it does: 
anyone who has ever been snorkling can testify to this. The deeper you go, 
the bluer the light gets. Flash pictures under water show far more color 
than the naked eye perceives. My recollection from a previous life when I 
was a physicist is that the factor at work is actually scattering, not 
absorption: in other words, the same mechanism that makes the sky blue. 
Your test showed no effect because your instrumentation was far too 
insensitive and/or the amount of water was far too small.

All that aside, I believe your conclusion is true: bodies of water seen 
from above look blue to us mostly because they are reflecting the blue sky, 
with some small added effect due to absorption or scattering. In IR photos, 
water looks dark mostly because it is reflecting the sky, again with some 
small added effect due to absorption or scattering.

Stan

*
****
*******
******************************************************
*  To remove yourself from this list, send:          *
*         UNSUBSCRIBE INFRARED                       *
*       to                                           *
*         MAJORDOMO@xxxxx                            *
*----------------------------------------------------*
*   For the IR-FAQ, IR-Gallery and heaps of links:   *
*  http://www.a1.nl/phomepag/markerink/mainpage.htm  *
******************************************************