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Previous Year CBSE Questions Page 291 of 484 
 
All the rays from the object refracted by the objective go through the eyering. Therefore, it is an 
ideal position for our eyes for 
viewing. If we place our eyes too 
close to the eye-piece, we shall not 
collect much of the light and also 
reduce our field of view. If we 
position our eyes on the eyering and 
the area of the pupil of our eye is 
greater or equal to the area of the 
eyering, our eyes will collect all the light refracted by the objective. The exact location of the 
eyering depends on the separation between the objective and the eyepiece and the focal length of 
the eyepiece. 
43. 
 
Magnifying power: The magnifying power of a telescope is defined as the ratio of the angle 
subtended at the eye by the final image formed at the least distance of distinct vision to the angle 
subtented at the eye by the object at infinity, when seen directly. 
Factors for increasing the magnifying power: 
(i) Increasing focal length of the objective 
(ii) Decreasing focal length of the eyepiece. 
Limitations of a refracting telescope: 
(v) Suffers from chromatic aberration 
(vi) Suffers from spherical aberration 
(vii) Small magnifying power 
(viii) Small resolving power 
Previous Year CBSE Questions Page 292 of 484 
 
Advantages of a reflecting telescope: 
(v) No chromatic aberration, because mirror objective is used. 
(vi) Spherical aberration can be removed by paraboloidal mirror. 
(vii) Image is bright because there is no loss of energy due to refraction. 
(viii) Large mirror provides an easier mechanical support over its entire back surface. 
44. 
 
Magnifying power, 𝑚 =
𝛽
𝛼
=
tan𝛽
tan𝛼
=
𝐴′𝐵′ 𝐴′𝐶2⁄
𝐴′𝐵′ 𝐴′𝐶1⁄
 
𝑚 = 
𝐴′𝐶1
𝐴′𝐶2
 
𝑚 = − 
𝑓0
𝑓𝑒
𝑤ℎ𝑒𝑟𝑒, 𝐴′𝐶1 = −𝑓0 , 𝐴′𝐶2 = 𝑓𝑒 
Construction difference: 
Microscope Telescope 
1. Objective is of very short focal length and 
of short aperture. 
1. Objective is of large focal length and of 
large aperture. 
2.Eyepiece is of short focal length (but fe > fo) 
and large aperture. 
2. Eyepiece is of short focal length and 
short aperture. 
 
Working difference: 
Microscope Telescope 
1. Objective forms real and magnified 
image of an object kept just beyond 
the focus. 
1. Objective forms image of the 
distant object at, or within, the 
focus of its eyepiece. 
2. It produces linear magnification, i.e., 
size of the image is larger than that of 
the object. 
2. It produces angular magnification 
i.e, the image is nearer to the eye 
but the size does not increase. 
45. (i) Refer 5 marks question, section B, question (5) 
(ii) 𝑓0 = 20𝑓𝑒 ∴
𝑓0
𝑓𝑒
= 20 ∴ 𝑚 = 20 
Previous Year CBSE Questions Page 293 of 484 
 
SECTION-C 
1MARK 
Q1. On what factors does the magnifying power of a compound microscope depend? 
Q2. What is the nature of final image formed in a compound microscope? 
Q3. What do you mean by normal adjustment of a telescope? 
Q4. The focal length of the objective and eyepiece of a telescope are 
𝑓𝑜 𝑎𝑛𝑑 𝑓𝑒respectively what is the magnifying power of the telescope in normal 
adjustment? 
Q5. Name the two types of reflecting telescopes? 
Q6. Write the important features of a good telescope? 
Q7. Define the term magnifying power of a compound microscope? 
Q8. Write the expression for magnifying power of a telescope when the final image is 
formed at the least distance of distinct vision 
Q9. In a telescope objective lens is of large focal length while eyepiece is of small focal 
length. Why 
Q10. Out of blue and red light which is deviated more by a prism? Give reason? 
MCQ 
Q11. To increase the magnifying power of a telescope, we should increase 
a) Focal length of the objective 
b) Focal length of the eyepiece 
c) Aperture of the objective. 
d) Aperture of the eyepiece. 
Q12. In a compound microscope, the objective produces a magnification of 10, while the 
eyepiece produces a magnification of 5, then the overall magnification achieved by a 
compound microscope is 
a) 2 b) 50 c) 0.5 d) 25 
Q13. If 𝑓𝑜and 𝑓𝑒 are the focal lengths of the objective and eyepiece of an astronomical 
telescope, the length of the tube is 
a) 𝑓𝑜+ 𝑓𝑒 
b) 𝑓𝑜- 𝑓𝑒 
Previous Year CBSE Questions Page 294 of 484 
 
c) √𝑓𝑜 × 𝑓𝑒 
d) 
𝑓𝑜𝑓𝑒
𝑓𝑜+ 𝑓𝑒
 
Q14. An astronomical telescope is set for normal adjustment and the distance between the 
objective and eyepiece is 1.05 metre. The magnifying power of the telescope is 20. 
What is the focal length of the objective? 
a) 2m b) 1m c) .5m d) .25m 
Q15. When a telescope is in normal adjustment, the distance of the objective from the 
eyepiece is 100cm. if the magnifying power of the telescope in normal adjustment is 
24, the focal length of the lenses are 
a) 96cm, 4cm b) 90cm, 10cm c) 80cm, 20cm d) 50cm, 50cm 
2MARKS 
Q16. Draw a ray diagram to show the image formation by a concave mirror when the object 
is placed between its focus and pole. Using this diagram, derive the magnification 
produced in the image. 
Q17. Draw a labelled diagram for the formation of image by a compound microscope when 
the final image is formed at the near point or the least distance of distinct vision? 
Q18. Draw a ray diagram to obtain the image at the least distance of distinct vision using an 
astronomical telescope? 
Q19 Draw a schematic ray diagram of a reflecting type telescope (Cassegrain)? 
Q20. Draw a ray diagram to obtain the image by an astronomical telescope in normal 
adjustment position? 
3MARKS 
Q21. Two convex lenses A and B of an astronomical telescope having focal lengths 5cm and 20 
cm respectively, are arranged as shown in the figure: 
(i) Which one of the two lenses you will select to use 
as the objective lens and why? 
(ii) What should be the change in the distance between 
the lenses to have the telescope in its normal 
adjustment position? 
(iii) Calculate the magnifying power of the telescope 
in the normal adjustment position. (CBSE SAMPLE PAPER 2003) 
Previous Year CBSE Questions Page 295 of 484 
 
Q22. Which two of the following lenses L1, L2 and L3 will you select as objective and eyepiece for 
constructing best possible (i) telescope (ii) microscope? Give reason to support your answer. 
(CBSE D 09, 15C; OD 17) 
Lenses Power (P) Aperture(A) 
L1 3 D 8cm 
L2 6 D 1 cm 
L3 10 D 1 cm 
 
Q23. Four double convex lenses, with the following specifications are available: 
Lenses Focal length Aperture(A) 
A 100 cm 10 cm 
B 100 cm 5 cm 
C 10 cm 2cm 
D 5 cm 2 cm 
 
Which two of the given four lenses, should be selected as the objective and eyepiece to construct 
an astronomical telescope and why? What will be the magnifying power and normal length of the 
telescope tube so constructed? 
Write the advantages of reflecting type telescope over such a telescope. 
Or 
Which two of the above four lenses should be selected as objective and eyepiece of a compound 
microscope and why? How can the magnifying power of such a microscope be increased? Draw a 
labelled ray diagram for the image formation in such a microscope. (CBSE SAMPLE PAPER 
2005) 
Q24. An optical instrument uses eye-lens of power 12.5 D and object lens of power 50D and has 
a tube length of 20 cm. Name the optical instrument and calculate its magnifying power, if it 
forms the final image at infinity. (CBSE D 17) 
Q25. A plot, between the angle of deviation (δ) and 
angle of incidence (i), for a triangular prism is shown 
in figure: 
Previous Year CBSE Questions Page 296 of 484 
 
Explain why any given value of ‘δ’ corresponds to two values of angle of incidence. State the 
significance of point P on the graph. (CBSE SAMPLE PAPER 2011) 
Q26. Define power of the lens. Why is the power of a lens measured as the reciprocal of its focal 
length? Give its SI unit and define it. Sun glasseshave curved surfaces but they do not have any 
power. Why? 
5MARKS 
Q27. (a) Derive mirror equation for a convex mirror. 
(b) Using it, show that a convex mirror always produces a virtual image, independent of the 
location of the object. (CBSE SP 20) 
Q28.Draw a ray diagram showing the formation of the image by a point object on the principal 
axis of a spherical convex surface separating two media of refractive indices n1 and n2, when a 
point source is kept in the rarer medium of refractive index n1. Derive the relation between object 
and image distance in terms of refractive index of the medium and radius of curvature of the 
surface. Hence obtain the expression for Lens Maker’s formula in the case of thin convex lens. 
 (CBSE D 09, 14, 14C, OD 16) 
Q29. Draw a ray diagram to show the working of a compound microscope. Deduce an expression 
for the total magnification when the final image is formed at the near point. 
In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal 
length 1.25 cm. If the eye piece has a focal length of 5 cm and the final image is formed at the near 
point, estimate the magnifying power of the microscope. (CBSE D10) 
Q30.(a) Draw a ray diagram for final image formed at distance of distinct vision (D) by a 
compound microscope and write an expression for its magnifying power. 
(b) An angular magnification of 30X is desired for a compound microscope using an objective of 
focal length 1.25 cm and eyepiece of focal length 5cm. How will you set up the compound 
microscope? (CBSE SP 20) 
Q31.(i) What is meant by ‘normal adjustment’ in case of an astronomical telescope? Trace the 
paths of three rays from a distant object through an astronomical telescope in normal adjustment. 
 (ii)A small telescope has an objective lens of focal length 140 cm and an eye-piece of focal length 
5.0 cm.What is the(a) magnifying power of telescope for viewing distant objects when the 
telescope is in normal adjustment (i.e., when the final image is at infinity)? 
Previous Year CBSE Questions Page 297 of 484 
 
(b) the final image is formed at the least distance of distinct vision (D = 25cm)? (CBSE OD13) 
(c) What is the separation between the objective and eye lens when final image is formed at 
infinity? 
(d) If this telescope is used to view a 100m tall tower 3km away, what is the height of the image 
of the tower formed by the objective lens? 
(e) what is the height of the final image of the tower if it is formed at the least distance of distinct 
vision D = 25 cm. 
SECTION C-ANSWERS OF 1 MARK QUESTIONS 
1. M depends on the focal length of objective and eyepiece. 
2. Real, inverted and magnified with respect to object. 
3. Adjustment in which final image is formed at infinity. 
4. Magnifying power M= -
𝑓𝑜
𝑓𝑒
 
5. Newtonian type and Cassegrain type 
6. High magnifying power, large light gathering power and high resolving power. 
7. Magnifying power M=
𝑎𝑛𝑔𝑙𝑒 𝑠𝑢𝑏𝑡𝑒𝑛𝑑𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑖𝑚𝑎𝑔𝑒 𝑎𝑡 𝑡ℎ𝑒 𝑒𝑦𝑒
𝑛𝑔𝑙𝑒 𝑠𝑢𝑏𝑡𝑒𝑛𝑑𝑒𝑑 𝑏𝑦 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 𝑎𝑡 𝑡ℎ𝑒 𝑒𝑦𝑒
 when both are at the least 
distance of distinct vision. 
8. Magnifying power M= -
𝑓𝑜
𝑓𝑒
(1+
𝑓𝑒
𝐷
 ) 
9. For larger angular magnification 
10. Deviation produced by a prism is 𝑑 = (𝑛 − 1)𝐴 and refractive index n∝ 
1
𝜆2 . 
Since violet has lesser wavelength its refractive index is more, hence deviation is more for 
violet. 
 ANSWERS TO MCQ 
11. a) Focal length of the objective 
12. a) 50 
13. a) 𝑓𝑜+ 𝑓𝑒 
14. b) 1m 
15. a) 96cm, 4cm 
 
 
Previous Year CBSE Questions Page 298 of 484 
 
ANSWERS TO QUESTIONS OF 2 MARKS 
16. 
 
 
 
17. 
 
18. 
 
Previous Year CBSE Questions Page 299 of 484 
 
19. 
 
20. 
 
 
3 MARKS QUESTION 
21. (i) The lens B should be used as objective because it has larger focal length and larger radius. 
(ii) In normal adjustment, the distance between objective and eyepiece 
= fo + fe = 20+5=25cm 
Distance required to be increased between the two lenses = 25-15 = 10 cm 
(iii) Magnifying power of the telescope in normal adjustment, 
𝑚 =
𝑓0
𝑓𝑒
=
20
5
= 4 
22.(i) Telescope: L1 as objective and L2 as eyepiece 
Reason: The objective should have large aperture and large focal length while the eyepiece should 
have small aperture and small focal length. Then the light gathering power and magnifying power 
will be larger. 
(ii) Microscope: L3 as objective and L2 as eyepiece 
Previous Year CBSE Questions Page 300 of 484 
 
Reason: Both the lenses of the microscope should have short focal lengths and the focal length of 
the objective should be smaller than that of the eyepiece. Magnifying power will be larger for short 
focal lengths of objective and eyepiece. 
23. For constructing astronomical telescope, lens A should be used as objective because of its 
large focal length and large aperture. Lens D should be used as its eyepiece because of its small 
focal length and small aperture. 
𝑚 =
𝑓0
𝑓𝑒
=
100
5
= 20 
Normal length = 100+5 =105 cm 
(b)Advantages of a reflecting telescope: 
(a) No chromatic aberration, because mirror objective is used. 
(b) Spherical aberration can be removed by paraboloidal mirror. 
OR 
For constructing compound microscope, the lens D should be used as objective and C as the 
eyepiece because the lenses should have small focal length and the focal length of the objective 
should be smaller than that of the eyepiece. 
𝑚 = −
𝑣0
𝑓0
(1 +
𝐷
𝑓𝑒
) 
The magnifying power of a compound microscope can be increased by taking both 𝑓0 and 𝑓𝑒 
small. 
For ray diagram refer question 3 of 5marks questions, Section B 
 
24. The optical instrument is compound microscope. 
𝑓𝑒 =
1
𝑃𝑒
=
100
12.5
= 8 
𝑓0 =
1
𝑃0
=
100
50
= 2 
𝑚 =
𝐿
𝑓0
×
𝐷
𝑓𝑒
=
20
2
×
25
8
= 31.25 
25. In general, any given value of deviation δ, (except for i = e ) corresponds to two values i and 
e. This is expected from the symmetry of i and e as 
δ= i + e – A, i.e., δ remains the same if i and e are interchanged.