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# Sun gives light at rate of $\;1400 Wm^{-2}\;$ of area perpendicular to direction of light . Assume $\;\lambda\;$ (sun light )= $\;6000 A^{0}\;.$ Then no.of Photons emitted from sun/sec assuming average radius of earth's orbit is $\;1.49\times10^{11} m\;$ is

$(a)\;2\times10^{36}\qquad(b)\;1.17\times10^{45}\qquad(c)\;1.96\times10^{72}\qquad(d)\;1.23\times10^{31}$

Answer : (b) $\;1.17\times10^{45}$
Explanation :
Total energy emitted / second = power (watts)
$\large\frac{\eta}{sec}=\large\frac{Power\;of\;sun\;(W)}{\large\frac{\varepsilon}{photon}}=\large\frac{I\times(4 n R^{2})\times(6000\times10^{-10})}{6.6\times10^{-34}\times3\times10^{8}} \quad \;$ (R average radius of earth's orbit)
$=1.178\times10^{45}\;.$