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STD 12 - 9. Ray optics abd optical instruments — NEET STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceNEETSTD 12 - 9. Ray optics abd optical instruments4 Marks
MCQ
Shown in the figure here is a convergent lens placed inside a cell filled with a liquid. The lens has focal length $+ 20\, cm$ when in air and its material has refractive index $1.50$. If the liquid has refractive index $1.60$, the focal length of the system is......$cm$
  • A
    $+ 80 $
  • B
    $-80$
  • C
    $-24$
  • ✓
    $-100$
✓

Answer

Correct option: D.
$-100$
d
(d) $\frac{1}{F} = \frac{1}{{{f_1}}} + \frac{1}{{{f_2}}} + \frac{1}{{{f_3}}}$

$\frac{1}{{{f_1}}} = (1.6 - 1)\left({\frac{1}{\infty } - \frac{1}{{20}}} \right) = - \frac{{0.6}}{{20}} = - \frac{3}{{100}}$…$(i)$

$\frac{1}{{{f_2}}} = (1.5 - 1)\left({\frac{1}{{20}} - \frac{1}{{ - 20}}} \right) = \frac{1}{{20}}$…$(ii)$

$\frac{1}{{{f_3}}} = (1.6 - 1)\left({\frac{1}{{ - 20}} - \frac{1}{\infty }} \right) = - \frac{3}{{100}}$…$(iii)$

$ \Rightarrow $ $\frac{1}{F} = - \frac{3}{{100}} + \frac{1}{{20}} - \frac{3}{{100}} \Rightarrow F = - 100$$cm$

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