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

Gujarat BoardEnglish MediumSTD 11 ScienceJEESTD 12 - 9. Ray optics abd optical instruments4 Marks
MCQ
A person is suffering from myopic defect. He is able to see clear objects placed at $15 \,cm$. What type and of what focal length of lens he should use to see clearly the object placed $60 \,cm$ away
  • ✓
    Concave lens of $20 \,cm$ focal length
  • B
    Convex lens of $20\, cm$ focal length
  • C
    Concave lens of $12\, cm$ focal length
  • D
    Convex lens of $12 \,cm$ focal length
✓

Answer

Correct option: A.
Concave lens of $20 \,cm$ focal length
a
( a) For viewing far objects, concave lenses are used and for concave lens

$u =$ wants to see $ = - \,60\,cm$;

$v =$ can see $ = - \,15\,cm$

so from $\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$$ \Rightarrow f = - \,20\,cm$.

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