5. A 10 cm tall object is placed perpendicular to the principal axis of a convex lens of focal
length 30 cm. The distance of the object from the lens is 20 cm. Find the:
(a)Position b)Nature (c)Size of the image formed.-
6.A lens is formed by combining two thin lenses of powers + 12 D and - 8 D in
contact with each other. What will be the focal length of combination ?
7. A convex lens has focal length of 20 cm. find its power in Dioptre.
8. The power of combination of two lenses separated by distance d
(a) decreases as their separation is increased
(b) increases as their separation is increased
(c) remains same
(d) becomes equal to the distance 'd'
9. Two converging lenses of equal focal lengths fare placed in contact. The
focal length of the combination is
(a) f
(b) 2f
(c) f/2
(d) 3f
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Answer:
Given
A 10cm tall object is placed perpendicularly to the axis of convex lens of focal length 30cm
To find
1)Position
2)Nature
3)Size of the image formed
Explanation
For a convex lens,
height of the object,ho=10cm
focal length=30cm
object distance=20cm
According to sign conventions,
u=-20 cm
f=30cm
v=?
f
1
=
v
1
−
u
1
30
1
=
v
1
−(
20
−1
)
=>
30
1
=
v
1
+
20
1
=>
v
1
=
30
1
−
20
1
=>
v
1
=
60
3−2
=>
v
1
=
60
1
=>
v=60cm
m=
u
v
=
h
o
h
i
\begin{lgathered}= > \dfrac{60}{-20} = \dfrac{h_i}{10} \\ \\ = > \boxed{\sf h_i =- 30cm}\end{lgathered}
=>
−20
60
=
10
h
i
=>
h
i
=−30cm
Image formed is Real, inverted and enlarged and formed at 30 cm on the other side of lens
Extra information
For concave mirror and biconcave lens
sign conventions are as follows
u=-u
f=-f
v=-v
unknown=+ve
For convex mirror and biconvex lens
sign conventions are as follows
u=-u
f=+f
v=+v
unknown=+ve