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Military research outfit Qinetiq has a way to stop snoopers reading sensitive documents with a sneaky sideways glance by printing them on a special spy-proof paper. It’s coated with closely packed strips of transparent material, each with a different refractive index from its neighbour. As reflected light passes from one strip to the next, they act as a diffraction grating that creates an iridescent effect. View the paper head-on, and light passes through only a few refractive index transitions, so whatever is printed on the coating is clearly visible (GB 2368405). But when the sheet is viewed sideways on, an…