$A$ thermopile is used for:

For a perfect black body,the absorption coefficient is

$Assertion :$ $A$ body that is a good radiator is also a good absorber of radiation at a given wavelength.
$Reason :$ According to Kirchhoff's law,the absorptivity of a body is equal to its emissivity at a given wavelength.

$A$ thermopile bolometer is used to detect:

For an opaque body,the coefficient of transmission is:

$A$ perfectly black body is placed in an enclosure (furnace) at room temperature. Then: