CBSE 2026 · Region 1 · Set 3 · Q28 · 3 marks
(a)Why does one prefer to view the image formed at infinity than that formed at near point in microscope/telescope ?(b)Consider lenses $\displaystyle \mathrm{L}_{1}, \mathrm{~L}_{2}$ and $\displaystyle \mathrm{L}_{3}$ as specified in the following table. Which of them will you select as objective and eyepiece for constructing best possible (i) telescope (ii) compound microscope ? Give reason for your answer. Lens $\displaystyle \mathrm{L}_{1}$ $\displaystyle \mathrm{L}_{2}$ $\displaystyle \mathrm{L}_{3}$ Power $\displaystyle 6$ D $\displaystyle 3$ D $\displaystyle 10$ D Aperture $\displaystyle 1$ cm $\displaystyle 8$ cm $\displaystyle 1$ cm
$\displaystyle 3$
(a)
Why does one prefer to view the image formed at infinity than that formed at near point in microscope/telescope ?
(b)
Consider lenses $\displaystyle \mathrm{L}_{1}, \mathrm{~L}_{2}$ and $\displaystyle \mathrm{L}_{3}$ as specified in the following table. Which of them will you select as objective and eyepiece for constructing best possible (i) telescope (ii) compound microscope ? Give reason for your answer.
$\displaystyle 3$
| Lens | $\displaystyle \mathrm{L}_{1}$ | $\displaystyle \mathrm{L}_{2}$ | $\displaystyle \mathrm{L}_{3}$ |
| Power | $\displaystyle 6$ D | $\displaystyle 3$ D | $\displaystyle 10$ D |
| Aperture | $\displaystyle 1$ cm | $\displaystyle 8$ cm | $\displaystyle 1$ cm |
Marking-scheme solution
(a)
In the case of normal adjustment, the eye muscles remain relaxed while they are strained in the case when image is formed at near point while viewing the object, hence it is preferred.
(b)
For Telescope
Lense $\displaystyle \mathrm{L}_{2}$ is suitable as objective Lens
Lense $\displaystyle \mathrm{L}_{3}$ is suitable as eyepiece
For Compound microscope
Lense $\displaystyle \mathrm{L}_{3}$ is suitable as objective lens
Lense $\displaystyle \mathrm{L}_{1}$ is suitable as eyepiece
Reason
For telescope, objective lens should have large aperture and large focal length
For microscope, objective and eye lens should have moderate aperture but the objective should have large power.
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CBSE Class 12 Physics past-paper question from the 2026board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.