FSc Part 1 Physics chapter 10: Optical instruments MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

17 exam-style questions on Optical instruments, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.

17 questions · 2 topics · 10 per test

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What chapter 10 covers

Ten Optical instruments questions with answers

Try each one, then open the answer. These ten stay the same; the test draws new ones every time.

1. The angular magnification of an optical instrument is the ratio of

  1. A
    image size to object size
  2. B
    image distance to object distance
  3. C
    the angle subtended at the eye by the image to the angle subtended by the object when placed at the near point and viewed directly
  4. D
    focal length of objective to that of eyepiece in every case
Show answer

Answer: C. Angular magnification M = θ'/θ compares the angle the image subtends with the angle the object subtends when seen by the unaided eye at 25 cm.

2. The least distance of distinct vision for a normal eye is about

  1. A
    10 cm
  2. B
    25 cm
  3. C
    50 cm
  4. D
    100 cm
Show answer

Answer: B. The near point of a normal eye is about 25 cm; objects closer than this cannot be focused clearly.

3. Compared with the object, the final image seen through a compound microscope is

  1. A
    real and erect
  2. B
    virtual and erect
  3. C
    virtual and inverted
  4. D
    real and inverted
Show answer

Answer: C. The objective forms a real inverted image; the eyepiece acts as a magnifier on it, producing a virtual image that stays inverted relative to the object.

4. A compound microscope has an objective of focal length 1.0 cm and an eyepiece of focal length 5.0 cm, with L = 16 cm. Using M = (L/fo)(1 + d/fe) with d = 25 cm, M is

  1. A
    80
  2. B
    96
  3. C
    21
  4. D
    400
Show answer

Answer: B. M = (16/1.0) × (1 + 25/5.0) = 16 × 6 = 96. Forgetting the 1 in the bracket gives 16 × 5 = 80.

5. The resolving power of a telescope can be increased by

  1. A
    decreasing the diameter of the objective
  2. B
    increasing the focal length of the eyepiece
  3. C
    using red light instead of blue light
  4. D
    increasing the diameter of the objective
Show answer

Answer: D. The least resolvable angle is about 1.22λ/D, so a larger aperture D (or shorter λ) lets finer detail be resolved. Red light has a longer wavelength, which makes it worse.

6. The refractive index of a glass is 1.5. The critical angle for light going from this glass into air is

  1. A
    41.8°
  2. B
    48.6°
  3. C
    33.7°
  4. D
    56.3°
Show answer

Answer: A. sin C = 1/n = 1/1.5 = 0.667, so C = 41.8°. 48.6° is the critical angle for water (n = 1.33).

7. In Michelson's rotating mirror experiment with an octagonal mirror, a steady image is seen when, in the time light travels to the distant mirror and back, the octagonal mirror turns through

  1. A
    one full revolution
  2. B
    half a revolution
  3. C
    one-eighth of a revolution
  4. D
    one-sixteenth of a revolution
Show answer

Answer: C. The next face must take the place of the previous one, which needs 1/8 of a turn. Time = 2d/c = 1/(8f), giving c = 16fd.

8. Which of these causes loss of signal power in an optical fibre?

  1. A
    total internal reflection at the core-cladding boundary
  2. B
    the high speed of light in glass
  3. C
    absorption and scattering of light by impurities and irregularities in the glass
  4. D
    the use of a lower-index cladding
Show answer

Answer: C. Power is lost when impurities absorb light and when tiny irregularities scatter it out of the core. Total internal reflection itself loses no energy.

9. An optical fibre keeps light inside its core by total internal reflection. For this to happen, the cladding must have

  1. A
    a higher refractive index than the core
  2. B
    the same refractive index as the core
  3. C
    a metallic reflecting coating
  4. D
    a lower refractive index than the core
Show answer

Answer: D. Total internal reflection occurs only when light in a denser medium meets a less dense one beyond the critical angle, so the cladding must be optically less dense.

10. In a multimode graded-index optical fibre,

  1. A
    the refractive index of the core increases towards its edge
  2. B
    the refractive index of the core decreases gradually from the centre outwards, reducing pulse spreading
  3. C
    the core is so thin (about 5 μm) that light follows only one path
  4. D
    the core and cladding have the same refractive index
Show answer

Answer: B. Rays travelling further from the axis move through lower refractive index and so travel faster, arriving together with the axial rays; this reduces dispersion of pulses.

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Ways to practise this chapter

Optical instruments: questions students ask

How many Optical instruments MCQs are there?

17, each with the answer and a short explanation, across 2 topics: Magnifying glass, microscopes and telescopes, Spectrometer, speed of light and optical fibres. Every chapter test draws ten of them, so you can take it many times.

Is the FSc Part 1 Physics chapter 10 test free?

Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.

How is the chapter test marked?

The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.

Can I read the explanations in Urdu?

Yes. In practice mode and on your results you can switch the explanations to Urdu.

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