Q: The sun radiates electromagnetic energy at the rate of 3.9 × 10^{26} W . Its radius is 6.96 × 10^{8} m . The intensity of sun light (in W/m^{2}) at the solar surface will be

(a) 5.6 × 10^{6}

(b) 5.6 × 10^{7}

(c) 4.2 × 10^{6}

(d) 4.2 × 10^{7}

Sol: $\large Intensity = \frac{P}{A}= \frac{3.9 \times 10^{26}}{4 \pi r^2} $

$\large = \frac{3.9 \times 10^{26}}{4 \times 3.14 \times (6.99 \times 10^8)^2} $

= 5.6 × 10^{7} W/m^{2}