physics : class12 Combination of two concave lenses : Combination of Thin Lenses in Contact : Optics MCQ Questions and Answers
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Combination of two concave lenses
MCQ questions and answers.
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MCQ Questions
1. Two concave lenses of powers -2 D and -3 D are placed in contact. What is their combined power?
A.
-5 D
B.
-1 D
C.
+1 D
D.
+5 D
Correct Answer:
-5 D
Explanation:
For thin lenses in contact, the combined power is the sum of their powers: P = -2 D + (-3 D) = -5 D.
2. Two concave lenses are placed in contact. If their focal lengths are -20 cm and -30 cm, what is the focal length of the combination?
A.
-12 cm
B.
-50 cm
C.
+12 cm
D.
+50 cm
Correct Answer:
-12 cm
Explanation:
The powers are -5 D and -3.33 D, approximately. Their total power is about -8.33 D, so the equivalent focal length is about -0.12 m, or -12 cm.
3. When two concave lenses are placed in contact, the combination behaves as which type of lens?
A.
A convex lens
B.
A concave lens
C.
A plane glass slab
D.
A cylindrical lens
Correct Answer:
A concave lens
Explanation:
Both concave lenses have negative powers. Their powers add to give a negative total power, so the combination behaves as a concave lens.
4. Two identical concave lenses, each having power -4 D, are placed in contact. What is the power of the combination?
A.
-8 D
B.
-4 D
C.
+4 D
D.
+8 D
Correct Answer:
-8 D
Explanation:
The combined power is the sum of the individual powers: P = -4 D + (-4 D) = -8 D.
5. The equivalent focal length of two concave lenses in contact is always:
A.
Positive
B.
Negative
C.
Zero
D.
Infinite
Correct Answer:
Negative
Explanation:
Each concave lens has negative focal length. Since their powers add to a negative value, the equivalent focal length is also negative.
6. A concave lens of power -5 D is placed in contact with another concave lens of power -1 D. What is the focal length of the combination?
A.
-16.7 cm
B.
-25 cm
C.
+16.7 cm
D.
+25 cm
Correct Answer:
-16.7 cm
Explanation:
The total power is -5 D + (-1 D) = -6 D. Therefore, f = 1/P = 1/(-6) m = -0.167 m, or approximately -16.7 cm.
7. If two concave lenses of powers -2 D and -6 D are placed in contact, what is the nature of the equivalent lens?
A.
Converging with power +4 D
B.
Diverging with power -8 D
C.
Converging with power +8 D
D.
Diverging with power -4 D
Correct Answer:
Diverging with power -8 D
Explanation:
The combined power is -2 D + (-6 D) = -8 D. A negative-power lens is diverging, so the combination is diverging with power -8 D.
8. Two concave lenses in contact have powers -3 D and -2 D. What is the magnitude of the power of their combination?
A.
1 D
B.
5 D
C.
6 D
D.
−5 D
Correct Answer:
5 D
Explanation:
Their combined power is -3 D + (-2 D) = -5 D. The magnitude of -5 D is 5 D.
9. For two concave lenses placed in contact, the equivalent power is found by:
A.
Subtracting their powers
B.
Multiplying their powers
C.
Adding their powers algebraically
D.
Dividing the larger power by the smaller power
Correct Answer:
Adding their powers algebraically
Explanation:
For thin lenses in contact, the equivalent power is P = P1 + P2. Since both lenses are concave, both powers are negative.
10. Two concave lenses in contact have an equivalent focal length of -0.5 m. What is the power of the combination?
A.
-2 D
B.
-0.5 D
C.
+0.5 D
D.
+2 D
Correct Answer:
-2 D
Explanation:
Using P = 1/f, with f = -0.5 m, the power is P = 1/(-0.5) = -2 D. The negative value is consistent with two concave lenses.
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