In which of the following phenomena do heat waves travel along straight lines with the speed of light?

If $25\%$ of the total incident radiation $Q = 500 \, J$ is absorbed and $105 \, cal$ is transmitted,calculate the percentage of reflected power.

$A$ piece of blue glass heated to a high temperature and a piece of red glass at room temperature are taken inside a dimly lit room. Then:

Inside a closed furnace held at a temperature of $400\, K,$ we have a black body. $A$ hole of area $10\, cm^2$ is opened in the furnace so that sunlight starts falling on the black body. The intensity of sunlight is $2000\, W/m^2.$ In the steady state:

$A$ body is in thermal equilibrium with its surroundings. This is because .....

The temperature of the sun is measured with: