Optics Ques 192
- Sunlight of intensity $1.3 $ $kWm^{-2}$ is incident normally on a thin convex lens of focal length $20 $ $cm$. 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 $22$ $ cm$ from the lens on the other side is
(2018 Adv.)
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Answer:
Correct Answer: 192.$(130)$
Solution:
$\Rightarrow \quad \frac{A _0^{\prime}}{A _0} =(\frac{2}{20}){ }^{2}=\frac{1}{100}$
$\Rightarrow \quad A _0^{\prime} =\frac{A _0}{100}$
$P =I _0 A _0=I _0{ }^{\prime} A _0{ }^{\prime}$
$\Rightarrow \quad I _0^{\prime} =\frac{I _0 A _0}{\frac{A _0}{100}}$
$=100 I _0=130 $ $kW / m^{2}$
BETA
