Total energy emitted per second by a unit area of a body is proportional to the fourth power of its absolute temperature. i.e. E=σT4 where, σ is a universal constant called Stefan Boltzmann constant. Given, T1=127∘C =127∘+273=400K T2=527∘C=800K and r1=8cm⇒r2=2cm We know that total energy. E=σAT4 In the first condition, E1=σ1A1T14 In the second condition. E2=σ2A2T24 On dividing Eq. (i) by Eq. (ii), we get ∣∴σ1=σ2] =