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enter image description here

This is a photo of a recentish book taken while backlit from a window. Notice that the pages have a slight yellow tinge to them, what I would call paper white. However you can see that the pages look very blue along the spine. This is not a photographic artifact, it is even more noticeable to the eye.

Can anyone explain why this would happen? I assume that the paper bleaching process might add some UV dies, but that seems to be something that would effect the whole page. Why does the color change just in this area?

UPDATE: the sunlight is from BEHIND the camera. The blue color is due to reflection not transmission. The outside of the book is very light grey. This effect happens with any book, go try it yourself.

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  • $\begingroup$ This seems more like a question about details of the bookbinding process than a question about physics. Do you have a particular reason for asking it here? $\endgroup$ – ACuriousMind Oct 7 '18 at 18:39
  • $\begingroup$ What color is the outside of the spine? Is it blue? If so then it's probably just sunlight getting tinged blue as it passes through the spine. $\endgroup$ – tparker Oct 7 '18 at 18:39
  • $\begingroup$ Curious, I believe this effect is due to the geometry. As you flatten it out the color goes away. Multiple reflection in the partial corner reflector? An optics effect one way or the other. $\endgroup$ – Maury Markowitz Oct 7 '18 at 18:52
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paper used in books of modern manufacture usually contains chemicals called brighteners which fluoresce with bluish light when illuminated with light containing a UV component, like sunlight. this makes the sheet look bright and white even if it contains some contaminants like dirt that would otherwise cause the sheet to have a yellowish cast, which is considered objectionable.

down near the spine of the book, the light scattering off one sheet (which is slightly enriched in bluish light) is illuminating the adjacent sheet, which is itself emitting slightly bluish light, and each sheet then makes the other look more bluish.

You will find this effect goes away if the book is illuminated with light from a tungsten filament light bulb, which doesn't contain much UV component.

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  • $\begingroup$ I suspect this is the correct answer, but one of the things I did was put it under a tungsten lamp for just this reason, and it definitely showed the same effect. $\endgroup$ – Maury Markowitz Oct 9 '18 at 13:15
  • $\begingroup$ holy cow! now we have a definite mystery! $\endgroup$ – niels nielsen Oct 9 '18 at 18:08
  • $\begingroup$ @nielsnielsen Indeed. I think the main part of the mystery is why Maury has accepted an answer which they still claim is inconsistent with their observations. $\endgroup$ – Emilio Pisanty Oct 10 '18 at 14:40

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