FSc Part 1 Physics chapter 9: Physical optics MCQs with answers

By Team Megalecture · Updated 4 Nov 2026

20 exam-style questions on Physical optics, 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.

20 questions · 2 topics · 10 per test

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What chapter 9 covers

Ten Physical optics questions with answers

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

1. A diffraction grating has 4000 lines per centimetre. Its grating element d is

  1. A
    2.5 × 10−6 m
  2. B
    2.5 × 10−4 m
  3. C
    4.0 × 10−5 m
  4. D
    2.5 × 10−7 m
Show answer

Answer: A. d = 1 cm/4000 = 2.5 × 10−4 cm = 2.5 × 10−6 m. 2.5 × 10−4 is the value in cm, not m.

2. In Young's double slit experiment, λ = 600 nm, the slit separation is 0.50 mm and the screen is 1.0 m away. The fringe spacing is

  1. A
    0.12 mm
  2. B
    0.60 mm
  3. C
    12 mm
  4. D
    1.2 mm
Show answer

Answer: D. Δy = λL/d = 600 × 10−9 × 1.0/(0.50 × 10−3) = 1.2 × 10−3 m = 1.2 mm.

3. Which of these waves cannot be polarized?

  1. A
    sound waves in air
  2. B
    light
  3. C
    radio waves
  4. D
    X-rays
Show answer

Answer: A. Sound in air is longitudinal: the air vibrates along the direction of travel, so there is no transverse plane to select. All electromagnetic waves are transverse.

4. X-rays, not visible light, are used to study the arrangement of atoms in a crystal because

  1. A
    X-rays travel faster than visible light
  2. B
    X-rays are charged particles
  3. C
    crystals are transparent only to X-rays
  4. D
    the wavelength of X-rays is of the same order as the spacing between atomic planes
Show answer

Answer: D. Diffraction is noticeable when the wavelength is comparable with the spacing of the "grating". Atomic spacings are about 10−10 m, like X-ray wavelengths.

5. Far away from a point source of light, a small part of a spherical wavefront is

  1. A
    cylindrical
  2. B
    plane
  3. C
    a sphere of smaller radius
  4. D
    elliptical
Show answer

Answer: B. The radius of the wavefront becomes very large, so a small portion of it is almost flat: a plane wavefront.

6. Huygen's principle states that

  1. A
    light consists of tiny particles
  2. B
    the new wavefront is the surface passing through the centres of the secondary wavelets
  3. C
    wavefronts always travel at the speed of sound
  4. D
    each point on a wavefront acts as a source of secondary wavelets, and the new wavefront is the surface tangent to them
Show answer

Answer: D. Every point on a wavefront sends out secondary wavelets at the wave speed; the envelope (common tangent) of these wavelets is the new position of the wavefront.

7. In a Michelson interferometer using light of wavelength 600 nm, 200 fringes cross the field of view as the movable mirror is moved. The mirror has moved

  1. A
    0.03 mm
  2. B
    0.06 mm
  3. C
    0.12 mm
  4. D
    0.60 mm
Show answer

Answer: B. Each fringe corresponds to a mirror movement of λ/2, so L = mλ/2 = 200 × 300 nm = 6.0 × 10−5 m = 0.06 mm. Using L = mλ gives 0.12 mm.

8. X-rays are diffracted by crystal planes 0.20 nm apart. The first-order Bragg reflection occurs at a glancing angle of 15°. The wavelength of the X-rays is about (sin 15° = 0.26)

  1. A
    0.052 nm
  2. B
    0.39 nm
  3. C
    0.20 nm
  4. D
    0.10 nm
Show answer

Answer: D. 2d sin θ = nλ: λ = 2 × 0.20 × 0.26 = 0.10 nm. Leaving out the 2 gives 0.052 nm.

9. A grating has a line spacing of 2.0 × 10−6 m. With light of wavelength 600 nm, what is the highest order of maximum that can be seen?

  1. A
    2
  2. B
    3
  3. C
    4
  4. D
    6
Show answer

Answer: B. sin θ cannot exceed 1, so m ≤ d/λ = 2.0 × 10−6/6.0 × 10−7 = 3.3. The highest whole number is 3.

10. Diffraction of light through a slit is most noticeable when the width of the slit is

  1. A
    much larger than the wavelength
  2. B
    of the same order as the wavelength
  3. C
    made of glass instead of metal
  4. D
    lit by white light instead of monochromatic light
Show answer

Answer: B. Spreading is significant only when the gap is comparable with the wavelength. Visible light has λ below 1 μm, so the slit must be very narrow.

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Ways to practise this chapter

Physical optics: questions students ask

How many Physical optics MCQs are there?

20, each with the answer and a short explanation, across 2 topics: Wavefronts and interference of light, Diffraction and polarization. Every chapter test draws ten of them, so you can take it many times.

Is the FSc Part 1 Physics chapter 9 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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