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Dual nature of radiation and photoelectric effect: NEET MCQs with answers

77 questions on this topic of Dual Nature of Matter and Radiation (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. The energy of a photon of wavelength 550 nm is (take hc = 1240 eV nm):

  1. 2.25 eV
  2. 1.13 eV
  3. 22.55 eV
  4. 0.444 eV
Show answer and explanation

Answer: (A) 2.25 eV

E = hc/λ = 1240 eV nm / 550 nm = 2.25 eV.

Quick tip: E (eV) = 1240 / λ (nm).

NCERT Class 12, Chapter 11

medium

2. The threshold wavelength for photoelectric emission from a metal is 5000 Å. Photoelectrons will be emitted when the metal is illuminated by light from a:

  1. 1 W ultraviolet lamp (2540 Å)
  2. 50 W sodium lamp (5890 Å)
  3. 5 W helium-neon laser (6328 Å)
  4. 100 W infrared lamp
Show answer and explanation

Answer: (A) 1 W ultraviolet lamp (2540 Å)

For photoelectric emission, the incident wavelength λ must be less than or equal to the threshold wavelength λ₀ (5000 Å). Only the ultraviolet lamp has λ = 2540 Å < 5000 Å, enabling photoemission regardless of its lower power.

hard

3. Light of two different frequencies ν₁ and ν₂ (with ν₁ > ν₂) is incident successively on a photosensitive metal. The ratio of the maximum speeds of the emitted photoelectrons in the two cases is 2 : 1. The work function of the metal is given by:

  1. h(4ν₂ - ν₁) / 3
  2. h(ν₁ - 4ν₂) / 3
  3. h(2ν₁ - ν₂) / 3
  4. h(4ν₁ - ν₂) / 3
Show answer and explanation

Answer: (A) h(4ν₂ - ν₁) / 3

Kinetic energy is proportional to the square of maximum speed, so K₁ / K₂ = (v₁ / v₂)² = (2/1)² = 4. Since K₁ = hν₁ - W₀ and K₂ = hν₂ - W₀, we have (hν₁ - W₀) / (hν₂ - W₀) = 4. This gives hν₁ - W₀ = 4hν₂ - 4W₀, which simplifies to 3W₀ = 4hν₂ - hν₁, or W₀ = h(4ν₂ - ν₁) / 3.

medium

4. Consider the following two statements regarding the photoelectric effect: Statement I: Wave theory of light predicts that the maximum kinetic energy of photoelectrons should increase with the intensity of incident light. Statement II: Wave theory of light fails to explain the existence of a threshold frequency and the practically instantaneous emission of photoelectrons. In light of the above statements, choose the correct option:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

According to classical wave theory, greater intensity means larger electric field amplitude, which should impart greater energy to electrons regardless of frequency, and energy would be absorbed gradually over time over the wavefront. Hence, classical wave theory predicts K_max depends on intensity, and fails to explain threshold frequency and the instantaneous emission. Both statements are correct.

easy

5. Light of wavelength 800 nm consists of photons of energy (hc = 1240 eV nm):

  1. 0.645 eV
  2. 0.775 eV
  3. 15.5 eV
  4. 1.55 eV
Show answer and explanation

Answer: (D) 1.55 eV

E = hc/λ = 1240 eV nm / 800 nm = 1.55 eV.

Quick tip: E (eV) = 1240 / λ (nm).

NCERT Class 12, Chapter 11

hard

6. A monochromatic light beam of wavelength λ falls normally on a perfectly absorbing surface. If the total energy falling on the surface in time t is U, the momentum transferred to the surface during this time is:

  1. 2U / c
  2. U / c
  3. U c
  4. U / (2c)
Show answer and explanation

Answer: (B) U / c

For a completely absorbing surface, all incident photons are absorbed. The momentum of a photon is p = E/c. Therefore, the total momentum transferred to the surface is Δp = U / c. (For a completely reflecting surface, it would be 2U/c).

medium

7. In a photoelectric effect experiment, monochromatic light strikes a target metal. If the intensity of incident light is tripled while its frequency is kept constant, which of the following statements is correct?

  1. The saturation photocurrent increases by a factor of 3
  2. The maximum speed of emitted photoelectrons triples
  3. The stopping potential increases by a factor of 3
  4. Both the saturation current and stopping potential triple
Show answer and explanation

Answer: (A) The saturation photocurrent increases by a factor of 3

Stopping potential depends only on the frequency of incident radiation and the work function, so it remains unchanged. The saturation photocurrent is directly proportional to the incident intensity, so tripling the intensity increases the saturation current by a factor of 3.

easy

8. The work function of caesium is 2.14 eV. What is the threshold frequency for photoelectric emission from caesium? (Take Planck's constant h = 6.63 × 10⁻³⁴ J s and 1 eV = 1.6 × 10⁻¹⁹ J)

  1. 4.45 × 10¹⁴ Hz
  2. 6.28 × 10¹⁴ Hz
  3. 3.23 × 10¹⁴ Hz
  4. 5.16 × 10¹⁴ Hz
Show answer and explanation

Answer: (D) 5.16 × 10¹⁴ Hz

Threshold frequency ν₀ = Φ₀ / h. Converting the work function to Joules: Φ₀ = 2.14 × 1.6 × 10⁻¹⁹ J = 3.424 × 10⁻¹⁹ J. Dividing by Planck's constant: ν₀ = (3.424 × 10⁻¹⁹) / (6.63 × 10⁻³⁴) ≈ 5.16 × 10¹⁴ Hz.

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9. A metal surface has a work function of 2.2 eV. When it is illuminated by light of frequency 8.0 × 10¹⁴ Hz, what is the maximum kinetic energy of the emitted photoelectrons? (Take h = 6.63 × 10⁻³⁴ J s and 1 eV = 1.6 × 10⁻¹⁹ J)

  1. 1.11 eV
  2. 3.31 eV
  3. 0.82 eV
  4. 2.20 eV
Show answer and explanation

Answer: (A) 1.11 eV

Energy of the incident photon E = hν = (6.63 × 10⁻³⁴ × 8.0 × 10¹⁴) / (1.6 × 10⁻¹⁹) eV = 5.304 / 1.6 eV = 3.315 eV. The maximum kinetic energy is K_max = E - Φ₀ = 3.315 eV - 2.20 eV ≈ 1.11 eV.

medium

10. Given below are two statements: Statement I: Photoelectric emission is an instantaneous process, occurring within about 10⁻⁹ s or less after illumination. Statement II: According to classical wave theory, light energy is distributed continuously over the wavefront, so an electron requires a measurable time lag to accumulate sufficient energy to escape. In the light of the above statements, choose the most appropriate answer from the options given below:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
Show answer and explanation

Answer: (A) Both Statement I and Statement II are correct

Photoelectric emission occurs with no detectable time delay (approx 10⁻⁹ s or less). Classical wave theory predicted a significant time lag because wave energy would be shared over a vast number of surface electrons. Thus both statements are correct.

easy

11. Light of wavelength 350 nm consists of photons of energy (hc = 1240 eV nm):

  1. 3.54 eV
  2. 0.282 eV
  3. 35.43 eV
  4. 1.77 eV
Show answer and explanation

Answer: (A) 3.54 eV

E = hc/λ = 1240 eV nm / 350 nm = 3.54 eV.

Quick tip: E (eV) = 1240 / λ (nm).

NCERT Class 12, Chapter 11

easy

12. The energy of a photon of wavelength 350 nm is (take hc = 1240 eV nm):

  1. 35.43 eV
  2. 3.54 eV
  3. 1.77 eV
  4. 0.282 eV
Show answer and explanation

Answer: (B) 3.54 eV

E = hc/λ = 1240 eV nm / 350 nm = 3.54 eV.

Quick tip: E (eV) = 1240 / λ (nm).

NCERT Class 12, Chapter 11

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