Switchable mirrors exhibit spectacular changes in their optical properties as a function of hydrogen concentration.
Yttrium is a shiny metal with a very low light transmission.
If hydrogen is admitted to such a film the transmission remains low up to a concentration of about 2. A slight reddish transmission is then observed. When more hydrogen is admitted to the sample the transmission first decreases and then rises sharply if the hydrogen concentration is more than about 2.8. The sample is then yellow transparent. In combination with the high mobility of hydrogen in metals this offers unique opportunities for real-time observation of hydrogen migration.