To understand this filter, we must look at how atoms vibrate inside solid crystals. Ionic solids contain a repeating grid of positively and negatively charged atoms. When infrared light hits this crystal grid, it can make the charged atoms vibrate.
If the light wave matches the natural vibration speed of the crystal, something unusual happens. The crystal stops letting the light pass through. Instead, it reflects almost all of the light back like a shiny metal mirror.
This high reflection zone is called the Reststrahlen band. Different crystals have different natural vibration speeds. For example, sodium chloride vibrates at a different speed than sapphire.
Because of this, chemists can choose different crystals to bounce back different colors of invisible infrared light. A potassium bromide crystal will block one specific range. A calcium fluoride crystal will block another.
Chemists use these filters when studying chemicals with far-infrared light. A common student mistake is thinking the filter absorbs the light like dark glass. Instead, the filter bounces the unwanted light away.
This helps scientists isolate a very pure beam of light. They can then use this pure beam to measure how other chemicals react.