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Physics · 2025 · 5 marks
CBSE 2025 · Region 5 · Set 1 · Q33
(a) (i) Draw a ray diagram of a reflecting telescope (Cassegrain) and explain the formation of image. State two important advantages that a…
(i)
Draw a ray diagram of a reflecting telescope (Cassegrain) and explain the formation of image. State two important advantages that a reflecting telescope has over a refracting telescope.
(ii)
In a refracting telescope, the focal length of the objective is $\displaystyle 50$ times the focal length of the eyepiece. When the final image is formed at infinity, the length of the tube is $\displaystyle 102$ cm. Find the focal lengths of the two lenses.
(i)
Write any two advantages of a compound microscope over a simple microscope. Draw a ray diagram for the image formation at the near point by a compound microscope and explain it.
(ii)
A thin planoconcave lens with its curved face of radius of curvature R is made of glass of refractive index $\displaystyle \mathrm{n}_{1}$. It is placed coaxially in contact with a thin equiconvex lens of same radius of curvature of refractive index $\displaystyle \mathrm{n}_{2}$. Obtain the power of the combination lens.
Marking-scheme solution
(i)
The parallel rays from a distant object are reflected by a large concave mirror. These rays are then reflected by a convex mirror placed just before the focus of the concave mirror and are converged to a point outside the hole. The final image is viewed through the eye piece.
Advantages (any two):
1) No chromatic aberration.
2) Less spherical aberration.
3) Less mechanical support required.
4) Brighter image.
5) High resolving power.
6) High magnifying power.
(ii)
For image at infinity:
$\displaystyle f_{0}+f_{e}=L$
According to the question:
$\displaystyle f_{0}=50 \times f_{e}$
$\displaystyle f_{e}+50 f_{e}=102$
$\displaystyle f_{e}=2 \mathrm{~cm}$
$\displaystyle f_{0}=100 \mathrm{~cm}$
(i)
Advantages (any two):
1) Larger magnification
2) Brighter image
The lens nearest the object, called the objective, forms a real, inverted, magnified image of the object. This serves as the object for the second lens, the eye piece, which functions like a simple microscope and produces a final image which is enlarged and virtual.
(ii)
Power of plano-concave lens $\displaystyle =P_{1}=-\frac{\left(n_{1}-1\right)}{R}$
Power of convex lens $\displaystyle =P_{2}=\left(n_{2}-1\right)\left(\frac{2}{R}\right)$
$\displaystyle P=P_{1}+P_{2}$
$\displaystyle =\frac{\left(2 n_{2}-n_{1}-1\right)}{R}$
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CBSE Class 12 Physics past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.