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On several old editions of the Guinness Book of World Records, there are shimmering parts inside the cover that somehow worked like curved mirror surfaces, while being thin as a piece of paper. Any idea how this could work? Somehow, despite being paper thin, and flat on the surface, they were able to produce remarkable variation in light reflection angle. You could tell they were flat by feel, but also the matte reflection was like any other glossy hardcover. In this image that I grabbed from a youtube video, you can see both the curved mirror effect and the matte reflection that shows that the surface is as flat as any typical hardcover, with the typical ripples, so it's not like mirror flat, but certainly not curved on the surface in order to produce this effect.

Picture showing the matte reflection combined with the bizarre curved mirror reflections

Examples: 2006 edition 2007 edition 2008 edition 2009 edition 2010 edition

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pressed into the cover of that book is a very thin film of aluminum foil that bears on its surface a very large number of very tiny, uniformly-spaced (and in this case) curved grooves which diffract the light that bounces off them. Such a thing is called a diffraction grating and can also be formed from a thin plastic sheet by pressing the sheet against a metal form containing the grooves hard enough to emboss the groove pattern unto the plastic surface.

A diffraction grating can behave just like a prism, splitting a beam of white light into its constituent wavelengths. A search on diffraction grating will point you to all the details of how they work and what can be done with them, besides dressing up book covers and decorative stickers for kids to play with.

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  • $\begingroup$ I’m well aware of how diffraction gratings work, but I have never seen anything like a diffraction grating being used to make a flat surface act as a curved reflector. Diffraction gratings also are dispersive optical elements, yet in the uncolored portions, like in the lettering in the 2008 cover, there is clearly no such dispersion. Gratings still do have zero order reflection, but flat gratings appear as flat mirrors as far as that zero order is concerned. Why do you think this is a grating effect? $\endgroup$
    – Liam Clink
    Commented Aug 17, 2023 at 20:13
  • $\begingroup$ I have seen both pressed/replica diffraction gratings used in stickers and also coarser gratings used as decoration. I took a guess. $\endgroup$
    – niels nielsen
    Commented Aug 18, 2023 at 1:50

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