Assuming the sun to have a spherical outer surface of radius $r$, radiating like a black body at temperature $t^o C$, the power received by a unit surface, (normal to the incident rays) at a distance $R$ from the centre of the sun is
where $\sigma$ is the Stefan's constant.
AIPMT 2007,AIPMT 1995, Medium
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Power $P$ radiated by the sun with its surface temperature $(t+273)$ $K$ is given by $Stefan's$ Boltzmann law.
where $r$ is the radius of the sun and the sun is treated as a black body where $e=1.$
The radiant power per unit area received by the surface at a distance $R$ from the center of the sun is given by
$S = \frac{P}{{4\pi {R^2}}} = \frac{{\sigma 4\pi {r^2}{{\left( {t + 273} \right)}^4}}}{{4\pi {R^2}}} = \frac{{{r^2}\sigma {{\left( {t + 273} \right)}^4}}}{{{R^2}}}.$
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