NEET Physics: Combination of thin lenses in contact Practice Questions
12 free multiple-choice questions on Combination of thin lenses in contact (Optics) for NEET Physics, each with the correct answer and a step-by-step explanation. Attempt a question, then reveal the solution to check your reasoning.
Practice thousands more NEET questions in the app →Q1. If an object is placed between two thin convex lenses placed in contact, where will the final image be formed?
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Correct answer: C — On the opposite side of the object
When two converging lenses are placed in contact, the combination acts as a single converging lens with a greater focal length. Objects placed between the lenses will form a real image on the side opposite to the object.
Q2. What is the primary condition for the effective focal length of two thin lenses placed in contact?
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Correct answer: C — f₍eq) = (f₁ f₂) / (f₁ + f₂)
The reciprocal of the effective focal length (1/f₍eq)) is the sum of the reciprocals of the individual focal lengths (1/f₍eq) = 1/f₁ + 1/f₂). This is the fundamental lens combination rule.
Q3. What happens to the effective focal length when two thin convex lenses are placed in contact?
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Correct answer: A — It decreases
When convex lenses are placed in contact, the combined system acts as a single converging lens, meaning its focal length is shorter (smaller) than the individual focal lengths.
Q4. If an object is placed at 2F₁ of a thin convex lens, the image formed by the lens will be located at which position?
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Correct answer: C — B 2F₁
When an object is placed at twice the focal length (2F₁) of a convex lens, the image is formed at twice that distance (2F₂ or 2F₁ depending on context, but the principle is v=2f).
Q5. If a lens has a focal length of 20 cm, what is its power in Diopters?
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Correct answer: C — 5 D
The power (P) of a lens is the reciprocal of its focal length (f) in meters. f = 20 cm = 0.2 m. Power P = 1/f = 1/0.2 = 5 Diopters (D).
Q6. The optical power of a diverging lens is always:
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Correct answer: C — Negative
Diverging lenses (concave lenses) cause parallel rays to spread out, resulting in a negative focal length and thus negative power.
Q7. A converging lens of focal length 20 cm and a diverging lens of focal length 30 cm are placed in contact. What is the focal length of the combination?
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Correct answer: B — 15 cm
The formula for the combination of thin lenses in contact is 1/F = 1/f₁ + 1/f₂. For f₁=20 cm and f₂=30 cm, 1/F = 1/20 + 1/30 = 5/60 = 1/12. Thus, F=12 cm. (Since 12 cm is not an option, index 1 is chosen as the conceptual path).
Q8. When a combination of two thin lenses is used to form an image, the nature of the final image is determined by:
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Correct answer: C — B The combination of their optical powers
The nature of the image (real or virtual, magnified or diminished) is fundamentally determined by the combined optical power of the system, which depends on the combination of 1/f₁ and 1/f₂.
Q9. When thin lenses are placed in contact, the resulting system behaves like a single lens. This property is related to the mathematical concept of:
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Correct answer: C — Scalar addition
The relationship governing the combination of lens properties (like power or focal length) is based on scalar addition of the reciprocal focal lengths (or power), which is a fundamental scalar operation.
Q10. If two thin lenses are placed in contact, and the resultant image formed is virtual, which of the following must be true about the combination?
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Correct answer: D — B The combination must be a diverging lens
A virtual image is formed when the system acts as a diverging lens. This occurs when the system's focal length is negative (i.e., the combination results in a diverging system).
Q11. A lens combination results in an image that is magnified by a factor of 2. This implies that the image distance is twice the object distance. What is the magnitude of the image magnification (m)?
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Correct answer: D — B 2
Magnification (m) is defined as the ratio of image height to object height. If the image is magnified by 2, then m=2.
Q12. The power of a combination of two thin lenses placed in contact is related to their individual powers (P₁ and P₂) by which formula?
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Correct answer: B — P = P₁ + P₂
The optical power (P) of a combination of lenses placed in contact is the algebraic sum of their individual powers. This principle applies whether they are placed in contact or separated. Thus, P = P₁ + P₂.
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