Question

An illuminance of $2.56\times {10}^{5}lm/m^2$ is produced by the sun on the earth. The luminous flux of the sun is : Assume radius of the earth is $1.5\times {10}^{11}m$.

• A. $5.625\times {10}^{28}lm$
• B. $7.2\times {10}^{28}lm$
• C. $7.2\times {10}^{27}lm$
• D. $5.625\times {10}^{27}lm$

Answer 1

The correct option is B $7.2\times {10}^{28}lm$

We have luminous flux = $4\pi I$
Also $E=\frac{I}{r^2}⟹I=E{r}^2$
Hence luminous flux = $4\pi E{r}^2$
Given $E=2.56\times {10}^{5}lm/m^2$
Hence Luminous flux = $4\times 3.14\times 2.56\times {10}^5(1.5\times {10}^{11}{)}^2$
= $7.2\times {10}^{28}lm$