Which of the following is the correct device for the detection of thermal radiation?

According to Kirchhoff's law,the ratio of emissive power to absorptive power for all bodies is:

For a perfect black body,the absorption coefficient is

The velocity of heat radiation in vacuum is

$A$ 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?

Define thermal radiation and radiant energy and explain Prevost's theory of heat exchange.