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Re: [Phys-l] Sun's image due to reflection by not a pinhole.




I recently heard Bill Nye speak and he spoke about a phenomenon that I had
never noticed and have not been able to reproduce. He said that when you
are outside and cast a shadow that the shadow will have a blue hue to it
because of the color of the sky. He pointed out that it is really only
noticeable when the shadow is projected onto a white surface. He said to
go
outside in a white t-shirt and hold your arm at an angle so that it casts
a
shadow on the shirt and it will have a blue hue.

I have tried this and cannot see the blue.


I understand the theory behind why this would be true . . . but I have not
been able to reproduce the effect. Has anybody else been able to? Maybe
it's the southern California smog making the shadows have a reddish-brown
hue instead of blue (LOL)?


There are several reasons why this is unobservable.
1. Other nearby objects are reflecting colored light into the shadow
2. Color perception of pigments tends to be constant regardless of the
illumination.
3. Bill Nye might be fooling himself.
4. The blue color might be an artifact of our color vision.

This effect should be observable if you take a picture with a film camera,
and have made user effect 1 is not happening. Do it in an open field. So
if you take a picture of a shadow on Mars you should see this effect.

The receptors in the eye are wired so that if you illuminate the center you
get a + signal. If you illuminate a ring around the center you get a -
signal, and if you illuminate the whole receptor cluster you get 0 signal.
This means that you really only can see differences across edges. This
visual system converts everything into essentially line drawings. As a
result your brain can figure out the color of an object as if it were viewed
in white light, because it only looks at color and shading differences
across boundaries.

This was demonstrated by Land in the late 60s at an APS meeting. He showed
that a red piece of paper still looked red when the actual color reflected
from it was blue. He used a Mondrian collage of colored papers with a blue
light on one side, and a Red one on the other. (please forgive any
misremembering of specific details).

The 3 primary color model of vision actually only applies to mechanical
reproductions systems, and very limited experiments with human vision. It
does not apply to natural illumination. Of course this is not brought out
in the physics texts which present that the 3 color model is gospel truth.
I suspect the authors do not know the actual facts behind color vision.

So this brings up a possibility of 3. What you perceive is very much what
you expect to perceive. This has been shown by a variety of cognitive
experiments, and is evident when you do a demo and students see completely
the opposite from what you see. There is some possibility that Nye may have
slightly different color vision which makes the effect more pronounced.
After all there are actually 2 different sets of color receptors that you
can have. One of the colors (either blue or green) has one of 2 different
pigments according your inheritance. Did Nye make this judgement using a
photo? Some people are also more sensitive to color variations, and some
have absolutely no color vision.

I have a beautiful photo of a snow scene taken from my bedroom window in the
early morning. It clearly shows the shadows as having a much more bluish
cast. Film is very sensitive to absolute color. Digital cameras
deliberately flatten the intensity variations and may wash this effect out.

Since color vision is "almost" independent of illumination, the blueness of
the shadow may actually be perceptible without having a blue sky. There are
other factors which may come into this such as the detergents used to wash
the shirt. They have optical brighteners which fluoresce and can make
whites appear more so. Old fashioned laundry products included "bluing"
which deliberately added a slight bluish cast to counteract the yellow in
white fabrics.

So was Nye fooled? I don't know! Contact him and see if he knows about the
modern color theory and why some people do not perceive the effect. The 3
primary color theory behind the effect is a flawed model because it does not
take into account the "constancy of color vision". This constancy effect is
why it is difficult to set up good experiments of color mixing.

John M. Clement
Houston, TX