Radiation Questions

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.

Assuming the sun to have a spherical outer surface of radius $r$, radiating like a black body at temperature $t{}^{\circ }\text{C}$, the power received by a unit surface, (normal to the incident rays) at a distance $R$ from the centre of the sun is:

A black body is at $727{}^{\circ }\text{C}$. It emits energy at a rate that is proportional to:

A black body at $227^{\circ }\text{C}$ radiates heat at the rate of $7{\text{Cals/cm}}^2\text{s}$. At a temperature of $727^{\circ }\text{C}$, the rate of heat radiated in the same units will be:

The total radiant energy per unit area, normal to the direction of incidence, received at a distance $R$ from the centre of a star of radius $r$, whose outer surface radiates as a black body at a temperature $T\text{ K}$ is given by:

If the radius of a star is $R$ and it acts as a black body, what would be the temperature of the star, in which the rate of energy production is $Q$?

A piece of iron is heated in a flame. It first becomes dull red, then reddish yellow, and finally turns white hot. The correct explanation for this observation is possible using: