Question

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) σr²T⁴/R²

(b) σr²T⁴/4πR²

(c)  σr²T⁴/R⁴

(d) 4πσr²T⁴/R²

(where σ is Stefan's constant) 

Answer 1

Correct option (a) σr²T⁴/R²

Explanation:

According to the Stefan Boltzmann law, the power radiated by the star whose outer surface radiates as a black body at temperature T K is given by

P = σ4πr²T⁴

where, r = radius of the star

σ = Stefan's constant

The radiant power per unit area received at a distance R from the centre of a star is