Question

Sunlight of intensity $1.3kW{m}^{-2}$ is incident normally on a thin convex lens of focal length $20cm$. Ignore the energy loss of light due to the lens and assume that the lens aperture size is much smaller than its focal length. The average intensity of light (in $kW{m}^{-2}$), at a distance $22cm$ from the lens on the other side is

Answer 1

$\frac{r}{R}=\frac{2}{20}=\frac{1}{10}$
$\therefore$ Ratio of area $={\left(\frac{r}{R}\right)}^2=\frac{1}{100}$, $E_o=1.3$
$\frac{a}{A}=\frac{1}{100}$
Total energy incident $=E_o\times A=E\times a$
$E=E_o\left(\frac{A}{a}\right)$
$E=1.3\times 100=130kW/m^2$