Ch-11: Dual Nature of Radiation and Matter20 Questions
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1. The photoelectric effect is the phenomenon in which:
A. Electrons are emitted from a metal surface when light of suitable frequency falls on it
B. Light is emitted when electrons strike a metal surface
C. Metals become magnetic under light
D. Electrons absorb heat and expand
Answer: A. Electrons are emitted from a metal surface when light of suitable frequency falls on it
Explanation: The photoelectric effect refers to the emission of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency is incident upon it.
2. Einstein's photoelectric equation is given by:
A. KEmax = hν + φ0
B. KEmax = hν − φ0
C. KEmax = φ0 − hν
D. KEmax = hν × φ0
Answer: B. KEmax = hν − φ0
Explanation: Einstein explained the photoelectric effect using KEmax = hν − φ0, where hν is the energy of the incident photon and φ0 is the work function of the metal.
3. The work function of a metal is defined as:
A. The energy required to accelerate an electron across the metal
B. The minimum energy needed to just eject an electron from the metal surface
C. The total kinetic energy of all free electrons
D. The energy of incident photons
Answer: B. The minimum energy needed to just eject an electron from the metal surface
Explanation: Work function (φ0) is the minimum amount of energy required to liberate an electron from the surface of a given metal, and it varies from metal to metal.
4. The threshold frequency in the photoelectric effect is the frequency:
A. Above which no photoemission occurs
B. Below which no photoemission occurs, regardless of intensity
C. At which photoemission is always maximum
D. Equal to the frequency of visible light always
Answer: B. Below which no photoemission occurs, regardless of intensity
Explanation: Threshold frequency (ν0) is the minimum frequency of incident light needed to cause photoemission; below this frequency, no electrons are emitted no matter how intense the light.
5. Stopping potential in a photoelectric experiment is used to measure the:
A. Number of photoelectrons emitted
B. Maximum kinetic energy of emitted photoelectrons
C. Frequency of incident light directly
D. Work function directly without calculation
Answer: B. Maximum kinetic energy of emitted photoelectrons
Explanation: Stopping potential (V0) is the minimum retarding potential needed to stop even the most energetic photoelectrons, so eV0 gives their maximum kinetic energy.
6. The photoelectric current (for frequency above threshold) is directly proportional to:
A. The frequency of incident light
B. The intensity of incident light
C. The work function of the metal
D. The stopping potential
Answer: B. The intensity of incident light
Explanation: Photoelectric current depends on the number of photoelectrons emitted per second, which is directly proportional to the intensity (number of photons per second) of incident light.
7. The maximum kinetic energy of photoelectrons depends on the:
A. Intensity of incident light only
B. Frequency of incident light, not its intensity
C. Distance of the source from the metal
D. Area of the metal surface
Answer: B. Frequency of incident light, not its intensity
Explanation: According to Einstein's equation, maximum kinetic energy of photoelectrons depends only on the frequency of incident radiation and the metal's work function, independent of intensity.
8. The energy of a photon is given by:
A. E = hν
B. E = h/ν
C. E = mc²/h
D. E = hc
Answer: A. E = hν
Explanation: A photon's energy is directly proportional to the frequency of the associated electromagnetic radiation, given by Planck's relation E = hν, where h is Planck's constant.
9. The momentum of a photon of wavelength λ is given by:
A. p = hλ
B. p = h/λ
C. p = λ/h
D. p = h²/λ
Answer: B. p = h/λ
Explanation: Since a photon has energy E = hc/λ and E = pc for a massless particle, its momentum works out to p = h/λ.
10. The de Broglie wavelength associated with a moving particle of momentum p is:
A. λ = h/p
B. λ = p/h
C. λ = hp
D. λ = h²/p
Answer: A. λ = h/p
Explanation: De Broglie proposed that every moving particle has an associated wavelength given by λ = h/p, linking the wave and particle aspects of matter.
11. The de Broglie wavelength of an electron decreases when:
A. Its velocity decreases
B. Its momentum increases (e.g., accelerated through higher voltage)
C. It is at rest
D. Planck's constant increases
Answer: B. Its momentum increases (e.g., accelerated through higher voltage)
Explanation: Since λ = h/p, a larger momentum (achieved for instance by accelerating an electron through a higher potential difference) results in a smaller de Broglie wavelength.
12. The Davisson–Germer experiment provided experimental confirmation of:
A. The photoelectric effect
B. The wave nature of electrons through electron diffraction
C. The particle nature of light
D. Nuclear fission
Answer: B. The wave nature of electrons through electron diffraction
Explanation: The Davisson–Germer experiment observed diffraction of electrons scattered from a nickel crystal, directly confirming de Broglie's hypothesis of the wave nature of matter.
13. Which phenomenon provides strong evidence for the particle nature of light?
A. Interference of light
B. Diffraction of light
C. The photoelectric effect
D. Polarization of light
Answer: C. The photoelectric effect
Explanation: The instantaneous emission of photoelectrons and their energy's dependence on frequency (not intensity) can only be explained if light behaves as discrete energy packets (photons), supporting its particle nature.
14. Which phenomena provide strong evidence for the wave nature of light?
A. Photoelectric effect and Compton effect
B. Interference and diffraction
C. Emission of photoelectrons
D. Black body radiation only
Answer: B. Interference and diffraction
Explanation: Interference and diffraction patterns, which arise from superposition of waves, can only be explained by treating light as a wave, providing strong evidence for its wave nature.
15. The value of Planck's constant (h) is approximately:
A. 6.63 × 10⁻³⁴ J·s
B. 3 × 10⁸ J·s
C. 1.6 × 10⁻¹⁹ J·s
D. 9.1 × 10⁻³¹ J·s
Answer: A. 6.63 × 10⁻³⁴ J·s
Explanation: Planck's constant, a fundamental constant relating a photon's energy to its frequency, has an approximate value of 6.63 × 10⁻³⁴ joule-second.
16. The threshold wavelength (λ0) of a metal is related to its work function (φ0) by:
A. λ0 = hc/φ0
B. λ0 = φ0/hc
C. λ0 = hφ0/c
D. λ0 = c/(hφ0)
Answer: A. λ0 = hc/φ0
Explanation: Since threshold frequency corresponds to photon energy just equal to the work function, hν0 = φ0, and using c = ν0λ0, we get λ0 = hc/φ0.
17. The number of photoelectrons emitted per second from a metal surface depends on:
A. The frequency of incident light only
B. The intensity (number of incident photons per second) of light, above the threshold frequency
C. The work function only
D. The stopping potential only
Answer: B. The intensity (number of incident photons per second) of light, above the threshold frequency
Explanation: Once the incident frequency exceeds the threshold, the rate of photoelectron emission depends on how many photons strike the surface per second, i.e., the intensity of light.
18. The work function of a metal is typically expressed in units of:
A. Joule only
B. Electron-volt (eV)
C. Watt
D. Coulomb
Answer: B. Electron-volt (eV)
Explanation: Because work function values are very small in joules, they are conventionally expressed in electron-volts (eV), a convenient energy unit for atomic-scale phenomena.
19. A photocell (used in devices like automatic door openers and light meters) works on the principle of:
A. Thermionic emission
B. The photoelectric effect
C. Nuclear fission
D. Electromagnetic induction
Answer: B. The photoelectric effect
Explanation: A photocell converts incident light into an electrical signal by using the photoelectric effect: light striking its photosensitive cathode ejects electrons, producing a measurable current.
20. If the frequency of incident light is increased (keeping intensity constant, and above threshold), the stopping potential:
A. Decreases
B. Increases
C. Remains constant
D. Becomes zero
Answer: B. Increases
Explanation: Since eV0 = hν − φ0, increasing the frequency of incident light increases the maximum kinetic energy of photoelectrons, and hence the stopping potential required to halt them also increases.
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