Re: Chlorophyll absorption

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On  6 Apr 99 at 17:59, George L Smyth wrote:

> Willem-Jan Markerink wrote:
> > 
> > A nice article, in addition to the IR-reflection of chlorophyll:
> > 
> > xxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > FROM: Marcangelo Puccio <mpuccio@xxxxxxxxxxxxxxxxxxx>
> > SUBJECT: Re: Night Vision Green?
> > DATE: Mon, 05 Apr 1999 21:40:11 -0700
> > NEWSGROUPS: sci.optics,sci.physics
> > 
> > Jim Mansfield wrote:
> > 
> > > In article <37091C73.7E228490@xxxxxxxxxxx>,
> > > Erik Max Francis  <max@xxxxxxxxxxx> wrote:
> > > >Thomas Jaworowski wrote:
> > >
> > > [snip night vison goggles being green - I haven't a clue why they are]
> > >
> > > >I haven't heard of that, but the Sun's frequencies peak at green, so it
> > > >makes sense that we would see best at around that peak frequency.  Also,
> > > >it's awfully convenient of chlorophyll (and other such molecules) absorb
> > > >energy around the peak frequency, in order to maximize the amount of
> > > >available energy available for photosynthesis.
> > >
> > > Doesn't there seem to be something conflicting about plant leaves _looking_
> > > green (ie, reflecting green light) and their absorbing energy at the
> > > Sun's peak frequency of green? Chlorophyll has absorption maxima in
> > > the blue and, to a lesser extent, in the red.
> > >
> > > -Jim
> > >
> > > --
> > > Jim Mansfield                          Internet: Jim.Mansfield@xxxxxx
> > > National Research Council of Canada    Phone:    (204) 984-5191
> > > Institute for Biodiagnostics           Fax:      (204) 984-5472
> > > http://www.ibd.nrc.ca/~mansfield/
> > 
> > Yes - you are right.
> > 
> > Plants photosynthesize poorly at best if illuminated by light at around 530
> > nm.  The following numbers are rough, but peak absorption in the red end is at
> > around 650 nm for Chlorophyll-a and at around 670-680 for Chlorophyll-b.
> > Peaks in the blue end are at around 440 and 460, respectively, with absorption
> > minima between 500 and 600 nm.  Some plants have 'accessory pigments' that are
> > able to absorb light pretty well in the 500 nm range (i.e., carotenoid piments
> > like beta carotene).  These form part of an 'antenna complex' that channels
> > energy from these other wavelengths to the chlrorophyll molecules that are the
> > core of the photosystems.  Some aquatic 'plants' (algae, actually) have
> > radically different absorptions that reflect the fact that certain wavelengths
> > of light are absorbed or reflected from sea water (i.e., red and golden algae
> > or 'seaweeds').
> > 
> > Not that this is a photobiology newsgroup... just had to comment!
> > 
> > -Marco
> 
> But chlorophyll is not the reason for the strong reflectance of infrared
> radiation.  The strong reflection is due to the air pockets between the cells
> within the leaf.

My (announcing) slip, sorry....:-))
To make this complete, it's the strong translucence of chlorophyll
for IR that makes this internal reflection invisible to the eye (only 
the above mentioned surface reflections remain visible, predominantly 
green), and bright white for IR-film.

This air-pocket internal reflection is also why snow reflects
strongly in IR....Walter Clark even addressed this topic in his
book, see a chapter on my homepage:

http://www.a1.nl/phomepag/markerink/mainpage.htm (or directly ir&snow.htm)


--                 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
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