physics : class12 Reciprocal focal length relation : Combination of Thin Lenses in Contact : Optics MCQ Questions and Answers
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Reciprocal focal length relation
MCQ questions and answers.
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MCQ Questions
1. For two thin lenses of focal lengths f1 and f2 placed in contact, what is the relation for the equivalent focal length F?
A.
1/F = 1/f1 + 1/f2
B.
F = f1 + f2
C.
1/F = 1/f1 - 1/f2
D.
F = f1f2
Correct Answer:
1/F = 1/f1 + 1/f2
Explanation:
For thin lenses in contact, the reciprocal of the equivalent focal length equals the sum of the reciprocals of the individual focal lengths.
2. Two converging lenses in contact have focal lengths 20 cm and 30 cm. What is their equivalent focal length?
A.
10 cm
B.
12 cm
C.
25 cm
D.
50 cm
Correct Answer:
12 cm
Explanation:
Using 1/F = 1/20 + 1/30 = 5/60, we get F = 12 cm.
3. Two thin lenses in contact have focal lengths 10 cm and 20 cm. What is the reciprocal of their equivalent focal length?
8. A combination of two lenses in contact has an equivalent focal length of 10 cm. One lens has a focal length of 20 cm. What is the focal length of the other lens?
A.
10 cm
B.
20 cm
C.
30 cm
D.
40 cm
Correct Answer:
20 cm
Explanation:
Using 1/10 = 1/20 + 1/f2, we get 1/f2 = 1/20, so f2 = 20 cm.
9. Two lenses in contact have focal lengths +10 cm and -30 cm. What is their equivalent focal length?
A.
+15 cm
B.
+30 cm
C.
-15 cm
D.
-30 cm
Correct Answer:
+15 cm
Explanation:
Using 1/F = 1/10 + 1/(-30) = 2/30 = 1/15, the equivalent focal length is +15 cm.
10. A lens combination has powers +2 D and +3 D. What is the equivalent focal length of the lenses in contact?
A.
0.2 m
B.
0.5 m
C.
2 m
D.
5 m
Correct Answer:
0.2 m
Explanation:
The combined power is P = 2 + 3 = 5 D. Since F = 1/P, F = 1/5 m = 0.2 m.
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