physics : class12 Equal-Focal-Length Lenses : Combination of Thin Lenses in Contact : Optics MCQ Questions and Answers
Practice important
Equal-Focal-Length Lenses
MCQ questions and answers.
Page 1
of
1
|
10
MCQ Questions
1. Two identical converging thin lenses, each having a focal length of +20 cm, are placed in contact. What is the focal length of the combination?
A.
+5 cm
B.
+10 cm
C.
+20 cm
D.
+40 cm
Correct Answer:
+10 cm
Explanation:
For two equal-focal-length lenses in contact, their powers add: 1/F = 1/f + 1/f = 2/f. Hence F = f/2 = 20/2 = +10 cm.
2. Five identical thin lenses of equal focal length are placed coaxially in contact. If the focal length of the combination is +5 cm, what is the focal length of each lens?
A.
+1 cm
B.
+10 cm
C.
+20 cm
D.
+25 cm
Correct Answer:
+25 cm
Explanation:
For n identical lenses in contact, 1/F = n/f, so f = nF. Therefore, f = 5 × 5 cm = +25 cm.
3. Two identical diverging lenses, each with focal length −30 cm, are placed in contact. What is the equivalent focal length of the combination?
A.
−60 cm
B.
−30 cm
C.
−15 cm
D.
+15 cm
Correct Answer:
−15 cm
Explanation:
Using 1/F = 1/f + 1/f for equal focal lengths, 1/F = 2/(−30 cm), giving F = −15 cm.
4. Three identical converging lenses are placed in contact and produce an equivalent focal length of +8 cm. What is the focal length of each individual lens?
A.
+8 cm
B.
+16 cm
C.
+24 cm
D.
+32 cm
Correct Answer:
+24 cm
Explanation:
For three equal-focal-length lenses, F = f/3. Thus, f = 3F = 3 × 8 cm = +24 cm.
5. Two identical converging lenses, each of focal length +20 cm, are in contact. An object is placed 30 cm in front of the combination. Where is the image formed?
A.
10 cm in front of the combination
B.
15 cm behind the combination
C.
20 cm behind the combination
D.
30 cm behind the combination
Correct Answer:
15 cm behind the combination
Explanation:
The equivalent focal length is F = 20/2 = 10 cm. Using 1/F = 1/v − 1/u with u = −30 cm, 1/10 = 1/v + 1/30, giving v = +15 cm, so the image forms 15 cm behind the combination.
6. Two identical thin lenses in contact have a combined power of +8 D. What is the focal length of each lens?
A.
+0.0625 m
B.
+0.125 m
C.
+0.25 m
D.
+0.50 m
Correct Answer:
+0.25 m
Explanation:
For equal lenses, the power of each is half the combined power: P = 8/2 = 4 D. Therefore, f = 1/P = 1/4 m = +0.25 m.
7. Four identical diverging lenses placed in contact have a combined power of −12 D. What is the focal length of each lens?
A.
−0.0833 m
B.
−0.1667 m
C.
−0.3333 m
D.
−0.6667 m
Correct Answer:
−0.3333 m
Explanation:
The power of each equal lens is −12/4 = −3 D. Hence its focal length is f = 1/(−3) m = −0.3333 m.
8. Two identical converging lenses, each of focal length +12 cm, are placed in contact. An object is located 18 cm in front of the combination. What is the image distance?
A.
6 cm behind the combination
B.
9 cm behind the combination
C.
12 cm behind the combination
D.
18 cm behind the combination
Correct Answer:
9 cm behind the combination
Explanation:
The equivalent focal length is F = 12/2 = 6 cm. With u = −18 cm, 1/6 = 1/v + 1/18, so 1/v = 1/9 and v = +9 cm.
9. Three identical converging lenses, each with focal length +30 cm, are in contact. An object is placed 40 cm in front of the combination. What is the magnification produced?
A.
−1/2
B.
−1/3
C.
+1/3
D.
+1/2
Correct Answer:
−1/3
Explanation:
The equivalent focal length is F = 30/3 = 10 cm. Using 1/10 = 1/v + 1/40 gives v = 40/3 cm. Thus magnification m = v/u = (40/3)/(−40) = −1/3.
10. A combination consists of an unknown number n of identical converging lenses, each having focal length +30 cm. If its equivalent focal length is +10 cm, what is n?
A.
2
B.
3
C.
4
D.
5
Correct Answer:
3
Explanation:
For n identical lenses in contact, F = f/n. Therefore, n = f/F = 30/10 = 3.
Short Questions and Answers
Practice short questions and answers related to
Equal-Focal-Length Lenses
.
Short questions are currently unavailable for this page.