Q15. A concave lens of focal length 25 cm and a convex lens of focal length 20 cm are placed in contact with each other. What is the power of this combination? What is the focal length of the combination?
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Answer:
The net focal length of the combination is
f
1
=
f
1
1
+
f
2
1
=
+25
1
+
−20
1
=−
100
1
⟹f=−100cm
Power of combination=P=P
1
+P
2
=
f
1
1
+
f
2
1
P=
0.25
1
+
−0.20
1
=
25
100
−
20
100
=
100
400−500
=−
100
100
=−1
⟹P=−1D
Since power is negative, the combination is diverging.
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