The quality or property of allowing heat to pass through a material without being absorbed; the characteristic of being diathermal or heat-transparent.
From Greek 'dia-' (through) + 'thermē' (heat) + '-acy' (quality or state), formed in 19th-century physics to describe thermal properties of materials that transmit heat readily.
Glass is diathermal—it lets infrared radiation pass right through which is why greenhouses work, but it actually blocks certain wavelengths which is why you can't get a tan through window glass.
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