Two spheres of the same material have radii $1\ m$ and $4\ m$ and temperatures $4000\ K$ and $2000\ K$ respectively. The ratio of the energy radiated per second by the first sphere to that by the second is

Two spherical black bodies of radii $r_1$ and $r_2$ and with surface temperatures $T_1$ and $T_2$ respectively radiate the same power. Then the ratio of $r_1$ and $r_2$ will be:

If $A$ represents Boltzmann constant,$B$ represents Planck's constant and $C$ represents speed of light in vacuum,then the quantity having the dimensions of $A^4 B^{-3} C^{-2}$ is

If the temperature of the sun is doubled,the rate of energy received by the earth will be increased by a factor of:

If the temperature of a black body increases from $7^{\circ}C$ to $287^{\circ}C$,then what is the ratio of the rate of energy emission?

The operating temperature and emissivity of a tungsten filament are $2000 \ K$ and $0.3$,respectively. The surface area of the filament is $A \ cm^2$. If the power of the lamp is $25 \ W$,find $A$. (Given: $\sigma = 5.67 \times 10^{-8} \ W/m^2K^4$)