Re: [Phys-L] Sliding Door Sliders as Optical Filters?

[John Denker <jsd@av8n.com>]

On 8/30/23 8:21 AM, dgpolvani--- via Phys-l wrote:
> We recently moved into a retirement apartment which has a balcony sliding
> door with slider (vertical) blinds.  We have a northwest view out the
> sliding glass door.  The apartment is on the sixth floor.  In the afternoon,
> when the sun is in the west, if we close the sliders, they appear reddish as
> they rotate about a vertical axis due to AC air currents.  Normally, the
> plastic sliders are translucent and off-white in color.  My wife and I are
> curious about what causes the reddish effect.  My best guess is that they
> are acting as selective optical filters as they rotate in the sunlight.
> They selectively pass more red light and less blue light as they rotate.

To figure that out, more information is needed.

1) The setting sun is red, for the same reason the clear daytime sky
is blue. It may be that the blinds allow you to see the sun-color
without having to look directly at the sun. It could be as simple
as that.

If the question is why the setting sun is red and the sky is blue,
please re-ask the question. There's a lot of nifty physics in that.
At some level it depends on Rayleigh scattering, but there's more
to the story. If the atmosphere were less dense, the mid-day sky
would be black. If it were more dense, it would be white. Some guy
named Einstein wrote a nifty paper about this.

2) If that's not it, you should gather more information. There are at
least two angles that matter: The incident angle, i.e. the angle of
the sunlight relative to the surface of the slat, and the transmitted
angle, i.e. your line of sight relative to the surface of the slat.
It should be easy to change each of those, independently, over a wide
range. If there are thin-layer interference fringes involved (like a
morpho butterfly), or something more complicated like microprisms or
filters, that will leap out at you.

You can make these observations at night, using a nice bright white
flashlight, so you don't need to worry about sun/sky color issues.

3) Beware that the human eye is not a photometer. Reeeeally not. You
perception of a "white" thing in a scene depends not only on that
thing but also other things. Try googling for
   perception discounting the illuminant
which will lead you to things like this:
  https://en.wikipedia.org/wiki/Color_constancy