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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
    Commented Oct 7, 2018 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
    Commented Oct 7, 2018 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. $\endgroup$ Commented Oct 7, 2018 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. $\endgroup$ Commented Oct 9, 2018 at 13:15
  • $\begingroup$ holy cow! now we have a definite mystery! $\endgroup$ Commented Oct 9, 2018 at 18:08
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    $\begingroup$ @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. $\endgroup$ Commented Oct 10, 2018 at 14:40
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Not mentioned yet—but relevant—is the very high reflectivity of surfaces at slight angles, as mediated by the Fresnel equations:

Optics students rediscover this to their delight in the corridor outside their lecture hall, where the floor looks completely unreflective when looking down but turns effectively into a mirror when looking far down its length.

This effect contributes to directing reflected light from the spine to the viewer's eye.

It's a fun and easy experiment to look down the length of an object that you'd consider matte (but smooth). Watch the mirror emerge!

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  • $\begingroup$ It’s funny you should have to mention this - I read a lot about the wizard war in WWII and it is precisely this effect that led to the destruction of the German U-boat force. Radar reflecting off the surface of the water at shallow angles, aided by long range and low flying altitude, formed corner cubes with the conning tower and allowed long-range detection. I should have realized this at once. $\endgroup$ Commented Mar 16, 2023 at 13:11

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