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.
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.