$A$ hot and a cold body are kept in vacuum separated from each other. Which of the following causes a decrease in the temperature of the hot body?

$Assertion :$ $A$ hollow metallic closed container maintained at a uniform temperature can act as a source of black body radiation.
$Reason :$ All metals act as black bodies.

Thermal radiation travels at the speed of . . . . . . .

Three very large plates of the same area are kept parallel and close to each other. They are considered as ideal black surfaces and have very high thermal conductivity. The first and third plates are maintained at temperatures $2\ T$ and $3\ T$ respectively. The temperature of the middle (i.e.,second) plate under steady-state condition is

Which instrument is used to detect infrared rays?

Two thermometers $A$ and $B$ are exposed to sunlight. The bulb of $A$ is painted black,but that of $B$ is not painted. Which of the following statements is correct regarding this case?