Optics Puzzle—One Window, Three Seasons

6. November 2007
Comments (2)

Submitted by H.S. Kwok, OSA Fellow

Can anyone explain the following optical phenomenon?

I went to a garden in Guangdong over the weekend. They claim to have a window that can convert the outdoor garden scene from summer to autumn to winter depending on the position of the windows, which are made of blue tinted glass.

I’d like to share the three pictures that I took. The first one is the original summer scene, green trees and all.



The second was taken with one blue tinted glass window pane.



The scene does look wintery with the leaves appearing rusty in color and the white tops looking sort of like snow.

However, the most interesting is the third picture, taken with two overlapping blue tinted glass window panes. (The windows are of the sliding door type.)



The leaves turn red! The scene now looks like autumn. I cannot explain how the red color can come out with two blue filters. The leaves in the picture taken with two blue filters are redder than those in the picture taken with one blue filter. Strange. I saw it with my own eyes and took the pictures with a Canon DSC. Can someone please explain this?

H.S. Kwok, OSA Fellow
Hong Kong
 

2007-11 November, Miscellaneous Optics , ,

Comments

Hossein Alisafaee
12/18/2007 10:11:03 AM #
Hi,

I was thinking about your beautiful pictures were taken in Guangdong. My conclusion is as follows:

Blue tints are made of Prussian blue (PB) with chemical formula Fe7(CN)18(H2O)x where 14 ≤ x ≤ 16. Blue color of Prussian blue is associated with the energy of the transfer of electrons from Fe(II) to Fe(III). Orange-red light at 680 nm is absorbed, and the transmitted light appears blue as a result. So, it acts not as a narrow band pass filter (the only puzzle point!), but as a narrow absorbing filter. This can be proved just by looking at the sky in the picture. It is still white! Because the average over all the visible wavelengths are still close to white and there is no difference if a very narrow band is included in averaging or not.

Now it is simple to explain the appeared red color. The appeared red color has wavelengths in other regions of red spectrum that couldn't be absorbed by PB.

Finally, the phenomenon is doubled by doubling the windows.

Hossein Alisafaee
Thomas A. Germer
1/4/2008 7:44:44 PM #
In response to the Optics Puzzle: One Window, Three Seasons posed by H.S. Kwok, I have a relatively simple explanation:

The blue filter absorbs light mostly in the middle of the visible spectrum, with some transmittance Tb at blue wavelengths and a much larger transmittance Tr at deep red wavelengths.  When you view the scene with a single filter, you see Tb of the blue light and Tr of the red light.  Since your eye is much less sensitive to the red light, you don't notice that there is some red light there, but you do see the blue light.  However, when you view the scene with two filters, the blue light is now reduced by Tb^2, while the red light has  been reduced by a much smaller amount, Tr^2.  Now that the blue light has been reduced so much, you begin to sense the red light that has not been absorbed by the filter.

My late father pointed out this effect to me once. He had noticed that some blue cellophane transmitted red light if he folded it enough times.  The garden in Guangdong takes this effect one step further.

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