concave lens of focal length 25cm and convex lens of focal length 20 CM are placed in contact with each other what is a power of this combination and also calculate focal length of this combination
options are...
A. P = -4D, f=-4m
B. P= +ID, f = + 1 m
C. P=+5D,f = +5m
D. P=-1D,f= +- 1m
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Answer:
Solution
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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.
Option is (A).
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sorry friend I don't know
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