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 the combined focal length is 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.
+6 D
D.
-5 D
Correct Answer:
+5 D
Explanation:
The power of lenses in contact is the algebraic sum of their powers. Therefore, P = +2 D + +3 D = +5 D.
3. What is the nature of the combination formed by placing two convex lenses in contact?
A.
Converging
B.
Diverging
C.
Plane
D.
Reflecting
Correct Answer:
Converging
Explanation:
A convex lens has positive power. The sum of the positive powers of two convex lenses is positive, so their combination is converging.
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 focal length becomes half. Therefore, F = 40/2 = 20 cm.
5. A convex lens of focal length 10 cm is placed in contact with another convex lens of focal length 15 cm. What is the power of the combination?
A.
+10 D
B.
+15 D
C.
+16.67 D
D.
+25 D
Correct Answer:
+16.67 D
Explanation:
The powers are P1 = 1/0.10 = +10 D and P2 = 1/0.15 = +6.67 D. Hence, the combined power is approximately +16.67 D.
6. Two convex lenses in contact have focal lengths 25 cm and 50 cm. What is the focal length of their combination?
A.
16.7 cm
B.
25 cm
C.
37.5 cm
D.
75 cm
Correct Answer:
16.7 cm
Explanation:
Using 1/F = 1/25 + 1/50 = 3/50, we get F = 50/3 cm, which is approximately 16.7 cm.
7. The combined power of two convex lenses in contact is +8 D. If one lens has power +3 D, what is the power of the other lens?
A.
+2 D
B.
+5 D
C.
+11 D
D.
-5 D
Correct Answer:
+5 D
Explanation:
For lenses in contact, P = P1 + P2. Therefore, P2 = 8 D - 3 D = +5 D.
8. Two convex lenses of focal lengths 20 cm and 20 cm are placed in contact. What is the power of the combination?
A.
+2.5 D
B.
+5 D
C.
+10 D
D.
+20 D
Correct Answer:
+10 D
Explanation:
Each lens has power P = 1/0.20 = +5 D. The combined power is +5 D + +5 D = +10 D.
9. For two convex lenses of focal lengths f1 and f2 placed in contact, which expression gives their equivalent focal length F?
A.
F = f1 + 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 equivalent power is the sum of the individual powers. Since power is the reciprocal of focal length, 1/F = 1/f1 + 1/f2.
10. A combination of two convex lenses in contact has an equivalent focal length of 10 cm. What is its power?
A.
+0.1 D
B.
+1 D
C.
+10 D
D.
-10 D
Correct Answer:
+10 D
Explanation:
Focal length must be expressed in metres for calculating power. Since F = 10 cm = 0.10 m, P = 1/F = 1/0.10 = +10 D.