NEETAceToday's NEET Daily

Thin-lens and lens-maker formulae; power and lens combinations: NEET MCQs with answers

59 questions on this topic of Optics (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

Practise Optics freeAll Optics questions

Free with a Google sign-in; NEET Daily needs no sign-in at all.

easy

1. Two thin lenses of focal lengths +20 cm and -40 cm are placed in coaxial contact with each other. What is the power of the combination?

  1. -5.0 D
  2. -2.5 D
  3. +5.0 D
  4. +2.5 D
Show answer and explanation

Answer: (D) +2.5 D

The equivalent focal length F of two thin lenses in contact is given by 1/F = 1/f₁ + 1/f₂. Here, 1/F = 1/20 - 1/40 = 1/40 cm⁻¹, which gives F = +40 cm = +0.4 m. The power of the combination is P = 1/F = 1/(+0.4 m) = +2.5 D.

medium

2. An illuminated object and a screen are placed at a fixed distance D = 90 cm apart. A converging lens of focal length f placed between them forms a sharp real image on the screen at two different positions separated by a distance d = 30 cm. What is the value of f?

  1. 25 cm
  2. 22.5 cm
  3. 15 cm
  4. 20 cm
Show answer and explanation

Answer: (D) 20 cm

By the displacement method, the focal length of the lens is given by f = (D² - d²) / (4D). Substituting D = 90 cm and d = 30 cm gives f = (90² - 30²) / (4 × 90) = (8100 - 900) / 360 = 7200 / 360 = 20 cm.

medium

3. A convex lens of focal length 50 cm is kept in contact with a concave lens of focal length 25 cm. The power of the combination is:

  1. −2 D
  2. 25 D
  3. −0.02 D
  4. 6 D
Show answer and explanation

Answer: (A) −2 D

P = P₁ + P₂ = 100/50 + 100/(-25) = −2 D (focal lengths in cm, so P = 100/f).

easy

4. An equiconvex lens of focal length f is cut into two identical halves along a plane perpendicular to its principal axis (transverse cut). The focal length of each individual half is:

  1. f / 2
  2. f
  3. 2f
  4. 4f
Show answer and explanation

Answer: (C) 2f

For the original equiconvex lens, 1/f = (μ - 1)(1/R - (-1/R)) = 2(μ - 1)/R. When cut along the plane perpendicular to the principal axis, each resulting piece is a plano-convex lens with radii R₁ = R and R₂ = ∞. Its focal length f' satisfies 1/f' = (μ - 1)(1/R - 0) = (μ - 1)/R = 1/(2f), so f' = 2f.

medium

5. A convex lens of focal length 25 cm is kept in contact with a convex lens of focal length 50 cm. The power of the combination is:

  1. 6 D
  2. 2 D
  3. 75 D
  4. 0.06 D
Show answer and explanation

Answer: (A) 6 D

P = P₁ + P₂ = 100/25 + 100/(50) = 6 D (focal lengths in cm, so P = 100/f).

medium

6. An object is placed 100 cm in front of a thin convex lens of focal length 20 cm. The distance of the image from the lens is:

  1. 25 cm
  2. 80 cm
  3. 40 cm
  4. 16.7 cm
Show answer and explanation

Answer: (A) 25 cm

1/v − 1/u = 1/f with u = −100 cm: 1/v = 1/20 − 1/100, so v = +25 cm (real image on the other side).

medium

7. A convex lens of focal length 50 cm is kept in contact with a convex lens of focal length 100 cm. The power of the combination is:

  1. 0.03 D
  2. 150 D
  3. 3 D
  4. 1 D
Show answer and explanation

Answer: (C) 3 D

P = P₁ + P₂ = 100/50 + 100/(100) = 3 D (focal lengths in cm, so P = 100/f).

medium

8. An object is placed 75 cm in front of a thin convex lens of focal length 25 cm. The distance of the image from the lens is:

  1. 50 cm
  2. 18.8 cm
  3. 75 cm
  4. 37.5 cm
Show answer and explanation

Answer: (D) 37.5 cm

1/v − 1/u = 1/f with u = −75 cm: 1/v = 1/25 − 1/75, so v = +37.5 cm (real image on the other side).

medium

9. An object is placed 37.5 cm in front of a thin convex lens of focal length 25 cm. The distance of the image from the lens is:

  1. 50 cm
  2. 12.5 cm
  3. 75 cm
  4. 15 cm
Show answer and explanation

Answer: (C) 75 cm

1/v − 1/u = 1/f with u = −37.5 cm: 1/v = 1/25 − 1/37.5, so v = +75 cm (real image on the other side).

medium

10. A convex lens of focal length 40 cm is kept in contact with a concave lens of focal length 50 cm. The power of the combination is:

  1. 0.005 D
  2. 4.5 D
  3. −10 D
  4. 0.5 D
Show answer and explanation

Answer: (D) 0.5 D

P = P₁ + P₂ = 100/40 + 100/(-50) = 0.5 D (focal lengths in cm, so P = 100/f).

medium

11. An object is placed 50 cm in front of a thin convex lens of focal length 25 cm. The distance of the image from the lens is:

  1. 50 cm
  2. 16.7 cm
  3. 100 cm
  4. 25 cm
Show answer and explanation

Answer: (A) 50 cm

1/v − 1/u = 1/f with u = −50 cm: 1/v = 1/25 − 1/50, so v = +50 cm (real image on the other side).

medium

12. A convex lens of focal length 50 cm is kept in contact with a concave lens of focal length 100 cm. The power of the combination is:

  1. −50 D
  2. 0.01 D
  3. 3 D
  4. 1 D
Show answer and explanation

Answer: (D) 1 D

P = P₁ + P₂ = 100/50 + 100/(-100) = 1 D (focal lengths in cm, so P = 100/f).

Keep going with Optics. All 503 questions with instant answers, your mistakes saved for revision and your accuracy by topic.
Start practising

Other topics in Optics