Switchable mirrors1-3 made of thin films of the hydrides of yttrium (YH(x)), lanthanum (LaH(x)) or rare-earth metals exhibit spectacular changes in their optical properties as x is varied from 0 to 3. For example, α- YH(x<0.23) is a shiny, hexagonally close-packed metal, β-YH(2±δ) is a face-centred cubic metal with a blue tint in reflection and a small transparency window at red wavelengths, whereas hexagonally close-packed γ- YH(x>2.85) is a yellowish transparent semiconductor. Here we show that this concentration dependence of the optical properties, coupled with the high mobility of hydrogen in metals, offers the possibility of real-time visual observation of hydrogen migration in solids. We explore changes in the optical properties of yttrium films in which hydrogen diffuses laterally owing to a large concentration gradient. The optical transmission profiles along the length of the film vary in such a way as to show that the formation of the various hydride phases is diffusion-controlled. We can also induce electromigration of hydrogen, which diffuses towards the anode when a current flows through the film. Consequently, hydrogen in insulating YH(3-δ) behaves as a negative ion, in agreement with recent strong-electron-correlation theories4,5. This ability to manipulate the hydrogen distribution (and thus the optical properties) electrically might be useful for practical applications of these switchable mirrors.@en