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  • A concave mirror is placed at the bottom of an empty tank with face upwards and axis vertical. When sunlight falls normally on the mirror, then it is focused at distance of <img alt="\mathrm{32\ cm

A concave mirror is placed at the bottom of an empty tank with face upwards and axis vertical. When sunlight falls normally on the mirror, then it is focused at distance of \mathrm{32\ cm} from the mirror. If the tank filled with water \mathrm{\left ( \mu =\frac{4}{3} \right )} upto a height of \mathrm{20\ cm}, then the sunlight will now get focused at:

Option: 1

\mathrm{16\ cm} above water level


Option: 2

\mathrm{9\ cm} above water level


Option: 3

\mathrm{24\ cm} below water level


Option: 4

\mathrm{9\ cm} below water level


Answers (1)

In first case, when sun is at infinity i.e.,

\mathrm{\begin{aligned} & \therefore \quad \mathrm{u}=\infty \\ & \Rightarrow \frac{1}{\mathrm{f}}=\frac{1}{-32}+\frac{1}{(-\infty)} \\ & \therefore \mathrm{f}=-32 \mathrm{~cm} \end{aligned}}
When water is filled in that tank upto a height of \mathrm{20\ cm}, the image formed by the mirror will act as virtual object \mathrm{O} for water surface.
\mathrm{\therefore \quad \mathrm{BI}=\mathrm{BO} \times \frac{3}{4}=12 \times \frac{3}{4}=9 \mathrm{~cm}}

Posted by

Ramraj Saini

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