Question

18
Cri
1.
10. A concave lens of focal length 20 cm from an
image at a distance of 10 cm from the lens. What
is the distance of the object from the lens?
[Ans: 20 cm]
11. At what distance from a convex lens of focal
length 2.5 m should a boy stand so that his image
is half his height?
[Ans: -7.5
m]
An
Homework: (Convex and Concave Lens)
1.
A convex lens forms a real and inverted image of
a pencil at a distance of 40 cm from the lens. The
image formed is of the same size as the object.
Find the focal length and power of the lens. At
what distance is the pencil placed from the lens?
(Ans: f= 20, PD = 5D)
2.
An object is placed at a distance of 10 cm from a
convex lens of focal length 12 cm. Find the
position and nature of the image
(Ans. V=- - 60 cm)
3. A concave lens of focal length 20 cm and an
object of height 4 cm is kept in front of it. If the
object distance is 30 cm. Find the position and
height of the image.
(Ans: v=-12 cm, h' = 1.6 cm)
4. An object kept 60 cm from a lens given a virtual
image 20 cm in front of the lens. Find f = = ? and
type of lens.
-10
(Ans: f=-30 cm, P =
D)
3
5. A concave lens of focal length 15 cm form an image
10 cm from the lens. How far is the object placed
from the lens. Draw the ray diagram.
(Ans. u=- 30 cm)
6. An object is kept at 10 cm in front of a convex lens.
Its image is formed on the screen at 15 cm from the
lens. Calculate (1) the focal length of the lens (2)
the magnification produced by the lens.
(Ans: (1) 6 cm, (2) m =-1.5)
7. An object is kept at 60 cm in front of a convex lens
of focal length 15 cm. Find the image distance and
the nature of the image. Also find the magnification
produced by the lens.
(Ans: v = 20 cm, The image is real, inverted and
smaller than the object. m =
3
Power of lens:
12. The power of a convex lens is 2.5 dioptres. Find
its focal length.
(Ans: 40 cm)
13. If the power of a lens is – 4 dioptres, find its focal
length.
(Ans: - 25 cm, concave)
14. An object of height 2 m is kept at 30 cm from a
convex lens. Its real image is formed at 60 cm
from the lens. Find the focal length and power of
the lens.
(Ans: f=20 cm, P = 5 D)
15. An object of height 4 cm is kept in front of a
concave lens of focal length 20 cm. If the object
distance is 30 cm, find the position and the height
of the image.
(Ans: v=-12 cm, h2 = 1.6 cm)
Combined power of Lens :
16. Two convex lenses of equal focal lengths are kept
in contact with each other. If the power of their
combination is 20 D, find the focal length of each
convex lens.
(Ans: fi = 0.1 m.)
17. If a convex lens of focal length 10 cm and a
concave lens of focal length 50 cm are kept in
contact with each other, (i) what will be the focal
length of the combination? (ii) what will be the
power of the combination ? (iii) what will be the
behavior of the combination (behavior as
convex lens / concave lens)?
(Ans: (i) 0.125 m = 12.5 cm, (ii) 8D, (iii) The
focal length of the combination of the lenses is
positive. This shows that the combination will
behave as a convex lens.)
a
1
*****
8. A convex lens produces a real and inverted image
2.5 times magnified at a distance of 25 cm from the
lens. Calculate the focal length of the lens.
[Ans: 7.14 cm]
9. A converging lens form a real image at a distance
of 15 cm. Find the position of object of the focal
length of the lens is 10 cm. Also, find the
magnification. State the characteristics of the
image formed.
[Ans: 30 cm, - 0.5, real, inverted and
diminished Image)
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Answer 1

Answer:

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