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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 biconvex lens forms a real image of an object placed perpendicular to its principal axis. Suppose the radii of curvature of the lens tend to infinity. Then the image would
  • A
    Disappear
  • B
    Remain as real image still
  • ✓
    Be virtual and of the same size as the object
  • D
    Suffer from aberrations
✓

Answer

Correct option: C.
Be virtual and of the same size as the object
c
(c) $\frac{1}{f} = (\mu - 1)\left( {\frac{1}{{{R_1}}} - \frac{1}{{{R_2}}}} \right)$
For biconvex lens ${R_2} = - {R_1}$

$\therefore $ $\frac{1}{f} = (\mu - 1)\left( {\frac{2}{R}} \right)$
Given $R = \infty $ $\therefore $ $f = \infty $, so no focus at real distance.

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