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9. Ray optics abd optical instruments — Physics STD 12 Science — Question

CBSE BoardEnglish MediumSTD 12 SciencePhysics9. Ray optics abd optical instruments1 Mark
MCQ
If the distance between object and its two times magnified virtual image produced by a curved mirror is $15 \mathrm{~cm}$, the focal length of the mirror must be :
  • A
     $15 \mathrm{~cm}$
  • B
    $-12 \mathrm{~cm}$
  • ✓
     $-10 \mathrm{~cm}$
  • D
    $10 / 3 \mathrm{~cm}$
✓

Answer

Correct option: C.
 $-10 \mathrm{~cm}$
c
$ \mathrm{m}=2=\frac{-\mathrm{v}}{\mathrm{u}} $

$ 2=\frac{-(15-\mathrm{u})}{-\mathrm{u}} $

$ 2 \mathrm{u}=15-\mathrm{u} $

$ 3 \mathrm{u}=15 \Rightarrow \mathrm{u}=5 \mathrm{~cm} $

$ \mathrm{v}=15-\mathrm{u}=15-5=10 \mathrm{~cm} $

$ \frac{1}{\mathrm{f}}=\frac{1}{\mathrm{v}}+\frac{1}{\mathrm{u}} $

$ =\frac{1}{10}+\frac{1}{(-5)}=\frac{1-2}{10}=\frac{-1}{10} $

$ \mathrm{f}=-10 \mathrm{~cm}$

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