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

Answer: 130

$\frac{r}{R}=\frac{2}{20}=\frac{1}{10}$
$\therefore$ Ratio of area $=\frac{1}{100}$
Let energy incident on lens be $E$.
$\therefore$ Given $\frac{E}{A}=1.3$
So final, $\frac{E}{a}=??$
$E=A\times 1.30$
Also $\frac{a}{A}=\frac{1}{100}$
$\therefore$ Average intensity of light at
$22cm=\frac{E}{a}=\frac{A\times 1.3}{a}$
$=100\times 1.3=130kW/m^2$