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 $T \ K$ is given by
- A$\sigma \frac{r^2}{R^2} T^4$
- B$\frac{\sigma r^2}{4\pi R^2} T^4$
- C$\sigma \frac{r^4}{R^4} T^4$
- D$\sigma \frac{4\pi r^2}{R^2} T^4$
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$A$ black sphere has radius $R$ whose rate of radiation is $E$ at temperature $T$. If the radius is made $\frac{R}{3}$ and the temperature is made $3T$,the rate of radiation will be:
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