Three objects coloured black, grey and white can withstand hostile conditions up to $2800 °\mathrm{C}$. These objects are thrown into a furnace where each of them attains a temperature of $2000 °\mathrm{C}$. Which object will glow brightest? 

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 $T \mathrm{K}$ is given by

(where $\sigma$ is Stefan's constant)

A slab of stone of area 0.36 m² and thickness 0.1 m is exposed on the lower surface to steam at 100ºC. A block of ice at 0ºC rests on the upper surface of the slab. In one hour 4.8 kg of ice is melted. The thermal conductivity of slab is:-

(Given latent heat of fusion of ice 3.36 × 10⁵ J kg^–1)

Two conductors having thickness d₁ and d₂, thermal conductivity k₁ and k₂ are placed one above the another. Find the equivalent thermal conductance:-

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