$A$ body is said to be opaque to the radiation if ($a$,$r$,and $t$ are the coefficients of absorption,reflection,and transmission,respectively).

$A$ 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?

$A$ sphere,a cube,and a thin circular plate,all made of the same material and having the same mass,are heated to the same temperature of $200^{\circ} C$. When these are left in a room,which one reaches room temperature the fastest?

$A$ solid cylinder of length $L$ and radius $r$ is heated up to the same temperature as that of a cube of edge length $a$. If both have the same material,volume,and are allowed to cool under similar conditions,then the ratio of the amount of radiation emitted will be (Neglect radiation emitted from the flat surfaces of the cylinder).

The spectrum of black body radiation is a

The velocity of heat radiation in vacuum is