Q.If a = 0.72 and r = 0.24, then the value of $t_r$ is (A) 0.02
(B) 0.04
(C) 0.4
(D) 0.2

A diathermanous substance transmits heat (infrared) radiation through it; glass is the standard example among the options.

Substances that allow thermal (infrared) radiation to pass through them largely unabsorbed are called diathermanous substances (e.g. dry air, rock salt, glass to a good extent for shorter IR wavelengths). Substances that absorb thermal radiation instead of transmitting it are called athermanous (e.g. wood, iron, copper — all opaque metals/wo …