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Physics · 2026 · 4 marks
CBSE 2026 · Region 4 · Set 1 · Q30
An astronomical telescope consists of two converging lenses. One of them of large aperture and large focal length is called objective lens and the…
An astronomical telescope consists of two converging lenses. One of them of large aperture and large focal length is called objective lens and the other one, of smaller focal length and smaller aperture is called the eyepiece. It is used to see distant objects which are not seen clearly with naked eyes. The image formed by the objective lens acts as an object for the eyepiece and the final image produced by the eyepiece is magnified.
(i)
The images formed by the objective lens and the eyepiece are respectively :
(A)
virtual, real
(B)
real, virtual
(C)
virtual, virtual
(D)
real, real
(ii)
The magnification produced by the telescope does not depend upon the :
(A)
colour of light
(B)
focal length of objective lens
(C)
focal length of eyepiece
(D)
apertures of objective lens and eyepiece
(iii)
Which of the following statements is not correct for this telescope ?
(A)
The focal length of objective lens $\displaystyle \left(\mathrm{f}_{0}\right)$ is larger than the focal length of eyepiece $\displaystyle \left(\mathrm{f}_{\mathrm{e}}\right)$.
(B)
Its magnifying power can be increased by increasing the focal length of objective lens $\displaystyle \left(\mathrm{f}_{0}\right)$.
(C)
The distance between two lenses is more than $\displaystyle \left(\mathrm{f}_{\mathrm{o}}+\mathrm{f}_{\mathrm{e}}\right)$.
(D)
The magnifying power can be decreased by increasing the focal length of eyepiece.
(iv)
(a)
An astronomical telescope has objective lens and eyepiece of focal lengths $\displaystyle 80$ cm and $\displaystyle 4$ cm respectively. To view the image in normal adjustment, the lenses must be separated by a distance of :
(A)
$\displaystyle 84$ cm
(B)
$\displaystyle 76$ cm
(C)
$\displaystyle 20$ cm
(D)
$\displaystyle 320$ cm
OR
(b)
Consider the telescope described in question (iv) (a). Its magnifying power in normal adjustment will be :
(A)
$\displaystyle 320$
(B)
$\displaystyle 84$
(C)
$\displaystyle 76$
(D)
$\displaystyle 20$
Marking-scheme solution
(i)(B)
real, virtual
(ii)(D)
apertures of objective lens and eyepiece
(iii)(C)
The distance between the two lenses is more than $\displaystyle \left(\mathrm{f}_{0}+\mathrm{f}_{\mathrm{e}}\right)$
(iv)(A)
$\displaystyle 84$ cm OR (D) $\displaystyle 20$
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CBSE Class 12 Physics past-paper question from the 2026 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.