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Question (41)

QN0036312

Two thin lenses of powers +2.5 D and -1.0 D are placed in contact. Their combined power is

(A) +1.5 D
(B) +3.5 D
(C) -1.5 D
(D) -3.5 D

Correct Answer: A

Explanation:

For lenses in contact, P = P1 + P2 = 2.5 - 1.0 = +1.5 D.

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Question (42)

QN0036313

Two lenses of powers +3 D and +2 D are placed in contact. Their equivalent focal length is

(A) 10 cm
(B) 20 cm
(C) 30 cm
(D) 50 cm

Correct Answer: B

Explanation:

Combined power = 3 + 2 = 5 D. Therefore f = 1/5 m = 0.2 m = 20 cm.

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Question (43)

QN0036314

A combination of lenses has a net power of -2 D. Its equivalent focal length is

(A) +50 cm
(B) -50 cm
(C) +20 cm
(D) -20 cm

Correct Answer: B

Explanation:

f = 1/P = 1/(-2) m = -0.5 m = -50 cm.

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Question (44)

QN0036315

A convex lens of focal length 50 cm forms a real image twice the size of the object. The object distance is

(A) 25 cm
(B) 50 cm
(C) 75 cm
(D) 100 cm

Correct Answer: C

Explanation:

For a real image twice the size, m = -2. Using m = v/u and the lens formula gives u = -75 cm.

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Question (45)

QN0036316

A convex lens forms a real image at 60 cm with magnification -2. The object distance is

(A) -60 cm
(B) -30 cm
(C) +30 cm
(D) +60 cm

Correct Answer: B

Explanation:

m = v/u. Thus -2 = 60/u, giving u = -30 cm.

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Question (46)

QN0036317

A convex lens has focal length 12 cm and forms a real image of the same size as the object. The object is placed at

(A) 6 cm
(B) 12 cm
(C) 24 cm
(D) 36 cm

Correct Answer: C

Explanation:

A convex lens forms an image of the same size when the object is at 2F. Therefore u = 2 × 12 = 24 cm.

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Question (47)

QN0036318

When an object is placed at the principal focus of a convex lens, the emergent rays are

(A) converging
(B) diverging
(C) parallel
(D) reflected back

Correct Answer: C

Explanation:

When an object is placed at the principal focus of a convex lens, the refracted rays emerge parallel to each other.

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Question (48)

QN0036319

A lens has a focal length of -0.25 m. Its power and type are

(A) +4 D, convex
(B) -2 D, concave
(C) -4 D, concave
(D) +2 D, convex

Correct Answer: C

Explanation:

P = 1/f = 1/(-0.25) = -4 D. A negative focal length indicates a concave lens.

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Question (49)

QN0036320

A convex lens of focal length 25 cm and a concave lens of focal length -50 cm are placed in contact. The combined power is

(A) +2 D
(B) -2 D
(C) +6 D
(D) -6 D

Correct Answer: A

Explanation:

Powers are +4 D and -2 D. Therefore, combined power = +4 - 2 = +2 D.

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Question (50)

QN0036321

To obtain a real and inverted image using a convex lens, the object should generally be placed

(A) between the optical centre and focus
(B) exactly at the focus
(C) beyond the principal focus
(D) at the optical centre

Correct Answer: C

Explanation:

When the object is placed beyond the principal focus of a convex lens, a real and inverted image is formed.

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Question (41)

Two thin lenses of powers +2.5 D and -1.0 D are placed in contact. Their combined power is

Explanation:

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Question (42)

Two lenses of powers +3 D and +2 D are placed in contact. Their equivalent focal length is

Explanation:

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Question (43)

A combination of lenses has a net power of -2 D. Its equivalent focal length is

Explanation:

Link:

Question (44)

A convex lens of focal length 50 cm forms a real image twice the size of the object. The object distance is

Explanation:

Link:

Question (45)

A convex lens forms a real image at 60 cm with magnification -2. The object distance is

Explanation:

Link:

Question (46)

A convex lens has focal length 12 cm and forms a real image of the same size as the object. The object is placed at

Explanation:

Link:

Question (47)

When an object is placed at the principal focus of a convex lens, the emergent rays are

Explanation:

Link:

Question (48)

A lens has a focal length of -0.25 m. Its power and type are

Explanation:

Link:

Question (49)

A convex lens of focal length 25 cm and a concave lens of focal length -50 cm are placed in contact. The combined power is

Explanation:

Link:

Question (50)

To obtain a real and inverted image using a convex lens, the object should generally be placed

Explanation:

Link:

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