physics : class12 Combination of two convex lenses : Combination of Thin Lenses in Contact : Optics MCQ Questions and Answers
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Combination of two convex lenses
MCQ questions and answers.
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MCQ Questions
1. Two convex lenses of focal lengths 20 cm and 30 cm are placed in contact. What is the focal length of the combination?
A.
10 cm
B.
12 cm
C.
25 cm
D.
50 cm
Correct Answer:
12 cm
Explanation:
For two thin lenses in contact, 1/F = 1/f1 + 1/f2. Thus, 1/F = 1/20 + 1/30 = 1/12, so F = 12 cm.
2. Two convex lenses having powers +2 D and +3 D are placed in contact. What is the power of the combination?
A.
+1 D
B.
+5 D
C.
-1 D
D.
+6 D
Correct Answer:
+5 D
Explanation:
The power of two thin lenses in contact is the sum of their powers: P = P1 + P2 = 2 D + 3 D = +5 D.
3. The equivalent power of two convex lenses in contact is always:
A.
Zero
B.
Negative
C.
Positive
D.
Equal to the smaller power only
Correct Answer:
Positive
Explanation:
A convex lens has positive power. Therefore, the sum of the powers of two convex lenses in contact is positive.
4. Two identical convex lenses, each having focal length 40 cm, are placed in contact. What is the focal length of the combination?
A.
20 cm
B.
40 cm
C.
80 cm
D.
160 cm
Correct Answer:
20 cm
Explanation:
For identical lenses in contact, the combined power is doubled, so the equivalent focal length becomes half: F = 40/2 = 20 cm.
5. Two convex lenses in contact have focal lengths 10 cm and 15 cm. The focal length of their combination is:
A.
5 cm
B.
6 cm
C.
12 cm
D.
25 cm
Correct Answer:
6 cm
Explanation:
Using 1/F = 1/10 + 1/15 = 1/6, the equivalent focal length is F = 6 cm.
6. If two convex lenses of powers +4 D and +6 D are placed in contact, their equivalent focal length is:
A.
0.1 m
B.
0.2 m
C.
0.25 m
D.
10 m
Correct Answer:
0.1 m
Explanation:
The combined power is P = 4 D + 6 D = 10 D. Since F = 1/P, F = 1/10 m = 0.1 m.
7. For two convex lenses placed in contact, the equivalent focal length is:
A.
Greater than the focal length of both lenses
B.
Less than the focal length of each lens
C.
Equal to the sum of their focal lengths
D.
Always zero
Correct Answer:
Less than the focal length of each lens
Explanation:
The powers add when convex lenses are placed in contact, making the combined power greater than either individual power. Hence, the equivalent focal length is smaller than each individual focal length.
8. A convex lens of focal length 25 cm is combined with another convex lens of focal length 50 cm. What is the power of the combination?
A.
+2 D
B.
+4 D
C.
+6 D
D.
-6 D
Correct Answer:
+6 D
Explanation:
The powers are P1 = 1/0.25 = 4 D and P2 = 1/0.50 = 2 D. Therefore, the combined power is 4 D + 2 D = +6 D.
9. Two convex lenses in contact have equivalent focal length 20 cm. If one lens has focal length 60 cm, what is the focal length of the other lens?
A.
10 cm
B.
20 cm
C.
30 cm
D.
80 cm
Correct Answer:
30 cm
Explanation:
Using 1/20 = 1/60 + 1/f2, we get 1/f2 = 1/20 - 1/60 = 1/30. Therefore, f2 = 30 cm.
10. Two convex lenses of powers +5 D and +2 D are placed in contact. The combination behaves like a lens of power:
A.
+3 D
B.
+7 D
C.
-3 D
D.
+10 D
Correct Answer:
+7 D
Explanation:
For thin lenses in contact, the equivalent power is the algebraic sum of the individual powers: P = +5 D + +2 D = +7 D.