The earth receives on its surface radiation from the sun at the rate of 1400 W/m2.The distance of the centre of the sun from the surface of the earth is 1.5 x 1011 m and the radius of the sun is 7.0 x 108 m. Treating sun as a black body, it follows from the above data that its surface temperature is

According to Stefan's law, E=σT4Total surface area of the sun=4πRS2 .Therefore, the total energy radiated per second by the sun per unit solid angle          =−σT4×4πRs24π=σT4Rs2Let Res be the distance of earth from the sun. Hence, intensity of radiation of earthl=σT4Rs2/Res2∴ 1400=5.6×10−8T47.0×1081.5×10112T=5801K