How can a solid be thermally transparent?
Thermal cameras designed to image wavelengths between 7-14µm (which is the peak for blackbody radiation around "room" temperatures) use lenses made of germanium, zinc sulfide, zinc selenide, or various chalcogenide glasses. All of these lens materials are solid substances, but they are practically transparent to thermal radiation at the blackbody temperatures where they operate. How does this work?
Atoms and molecules contain electrons. Think of these electrons as being attached to the atoms by springs. The electrons have a natural frequency at which they tend to vibrate. When a light wave with that same natural frequency impinges upon an atom, then the electrons of that atom will be set into vibrational motion. If a light wave of a given frequency strikes a material with electrons having the same vibrational frequencies then those electrons will absorb the energy. Different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.
Light Reflection and Transmission
Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural frequencies of vibration of the objects, instead of vibrating in resonance at a large amplitude the electrons vibrate for brief periods of time with small amplitudes of vibration and the energy is reemitted as a light wave.
If the object is transparent then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object.
If the object is opaque then the vibrations of the electrons on the material's surface vibrate for short periods of time and then reemit the energy as a reflected light wave.
An ideal lens, for whatever frequency, transmits far more energy than it absorbs or reflects.
Absorption can especially be a problem with infrared lenses where the Coefficient of Thermal Expansion (CTE) of most IR materials is orders of magnitude higher than those of visible glasses, creating large changes in the refractive index.
Much as with an achromatic doublet, which uses a positive and negative element of different materials with equal and opposite amounts of chromatic aberration to correct for color, you would solve an athermal doublet equation to ensure that temperature changes are compensated for and focus is maintained. Calculations must also compensate for expansion and contraction of the lens body.
The use of materials more expensive than glass, difficult to work with (both grinding/polishing and the additional calculations) and the small market for such lenses tend to make them extremely expensive.