physics : class12 Equivalent focal length : Combination of Thin Lenses in Contact : Optics MCQ Questions and Answers
Practice important
Equivalent focal length
MCQ questions and answers.
Page 1
of
4
|
40
MCQ Questions
1. Two thin converging lenses of focal lengths 20 cm and 30 cm are placed in contact. What is their equivalent focal length?
A.
10 cm
B.
12 cm
C.
25 cm
D.
50 cm
Correct Answer:
12 cm
Explanation:
For thin lenses in contact, 1/F = 1/f1 + 1/f2. Thus, 1/F = 1/20 + 1/30 = 1/12, so the equivalent focal length is 12 cm.
2. Two lenses in contact have powers +2 D and +3 D. 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 equivalent power of lenses in contact is the sum of their powers: P = P1 + P2 = 2 D + 3 D = +5 D.
3. A combination of two thin lenses in contact has powers +4 D and -1 D. What is its equivalent power?
A.
+5 D
B.
+3 D
C.
-3 D
D.
-5 D
Correct Answer:
+3 D
Explanation:
For lenses in contact, equivalent power is the algebraic sum: P = +4 D + (-1 D) = +3 D.
4. The equivalent focal length of two thin lenses in contact is given by which expression?
A.
F = f1 + f2
B.
1/F = 1/f1 + 1/f2
C.
F = f1 - f2
D.
1/F = f1 + f2
Correct Answer:
1/F = 1/f1 + 1/f2
Explanation:
For two thin lenses in contact, the reciprocal of the equivalent focal length equals the sum of the reciprocals of their focal lengths.
5. Two identical thin lenses, each having focal length 20 cm, are placed in contact. What is the equivalent focal length?
A.
10 cm
B.
20 cm
C.
40 cm
D.
5 cm
Correct Answer:
10 cm
Explanation:
For identical lenses in contact, 1/F = 1/f + 1/f = 2/f. Therefore, F = f/2 = 20/2 = 10 cm.
6. A converging lens of focal length 25 cm is placed in contact with another converging lens of focal length 50 cm. What is the equivalent focal length?
A.
16.7 cm
B.
25 cm
C.
75 cm
D.
100 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. A lens of focal length +20 cm is placed in contact with a lens of focal length -20 cm. What is the equivalent focal length of the combination?
A.
10 cm
B.
-10 cm
C.
20 cm
D.
Infinite
Correct Answer:
Infinite
Explanation:
The powers are +5 D and -5 D, whose sum is zero. Hence the combination has zero power and its equivalent focal length is infinite.
8. Two thin lenses in contact have powers +5 D and +5 D. What is the equivalent focal length of the combination?
A.
5 cm
B.
10 cm
C.
20 cm
D.
50 cm
Correct Answer:
10 cm
Explanation:
The equivalent power is 5 D + 5 D = 10 D. Since F = 1/P, F = 1/10 m = 0.1 m = 10 cm.
9. A combination of thin lenses in contact has an equivalent power of -2 D. What is its equivalent focal length?
A.
-0.5 m
B.
+0.5 m
C.
-2 m
D.
+2 m
Correct Answer:
-0.5 m
Explanation:
Equivalent focal length is F = 1/P. Therefore, F = 1/(-2) m = -0.5 m.
10. A converging lens of focal length 10 cm is placed in contact with a diverging lens of focal length -20 cm. What is the equivalent focal length?
A.
+20 cm
B.
-20 cm
C.
+10 cm
D.
-10 cm
Correct Answer:
+20 cm
Explanation:
Using 1/F = 1/10 + 1/(-20) = 1/20, the equivalent focal length is F = +20 cm.