JSMU Karachi Entry Test Physics: Modern Physics MCQs

JSMU Karachi Entry Test Physics: Modern Physics MCQs — sample questions

1. Question 1

   Q1. What is the minimum energy required to eject an electron from a metal surface in the photoelectric effect?

   • A) Work function
   • B) Threshold energy
   • C) Ionization energy
   • D) Binding energy

   Answer: Work function

   Explanation: Work function is the minimum energy required; it is a characteristic of the metal, not the incident light.

2. Question 2

   Q2. The energy of a photon is given by which equation?

   • A) E = hf
   • B) E = mc^2
   • C) E = pv
   • D) E = 1/2mv^2

   Answer: E = hf

   Explanation: E = hf relates photon energy to frequency; 'h' is Planck's constant, 'f' is frequency.

3. Question 3

   Q3. What is the phenomenon where an X-ray photon is scattered by a free electron, resulting in a change in wavelength?

   • A) Compton effect
   • B) Photoelectric effect
   • C) Pair production
   • D) Bremsstrahlung

   Answer: Compton effect

   Explanation: Compton effect involves scattering; it demonstrates particle-like behavior of photons, unlike wave-like diffraction.

4. Question 4

   Q4. In the context of the Bohr model of the hydrogen atom, what is the term for the energy required to remove an electron from the ground state?

   • A) Ionization energy
   • B) Electron affinity
   • C) Binding energy
   • D) Excitation energy

   Answer: Ionization energy

   Explanation: Ionization energy is the energy needed to remove an electron; for hydrogen, it's 13.6 eV.

5. Question 5

   Q5. What is the term for the production of an electron-positron pair when a high-energy photon interacts with a strong magnetic field or a nucleus?

   • A) Pair production
   • B) Annihilation
   • C) Compton scattering
   • D) Bremsstrahlung

   Answer: Pair production

   Explanation: Pair production involves converting photon energy into matter; requires energy >= 1.022 MeV.

6. Question 6

   Q6. The half-life of a radioactive substance is a measure of its

   • A) Activity
   • B) Decay constant
   • C) Stability
   • D) Rate of decay

   Answer: Rate of decay

   Explanation: Half-life is the time for half the nuclei to decay; it's inversely related to the decay constant.

7. Question 7

   Q7. The de Broglie wavelength of a particle is related to its

   • A) Momentum
   • B) Energy
   • C) Frequency
   • D) Speed

   Answer: Momentum

   Explanation: de Broglie wavelength = h/p, where 'h' is Planck's constant and 'p' is momentum.

8. Question 8

   Q8. In an X-ray tube, the minimum wavelength (λ_min) of the emitted X-rays is related to the potential difference (V) between the cathode and anode by

   • A) λ_min = hc/eV
   • B) λ_min = eV/hc
   • C) λ_min = h/eV
   • D) λ_min = e/hV

   Answer: λ_min = hc/eV

   Explanation: λ_min = hc/eV is derived from the maximum energy (eV) of the electrons, which equals the photon energy (hc/λ).

9. Question 9

   Q9. The phenomenon of electron emission from a metal surface when light is shone on it is known as

   • A) Photoelectric effect
   • B) Thermionic emission
   • C) Field emission
   • D) Secondary emission

   Answer: Photoelectric effect

   Explanation: Photoelectric effect involves light-induced electron emission; energy depends on light frequency, not intensity.

10. Question 10

    Q10. The energy levels in a hydrogen atom are given by the equation E_n = -13.6 eV/n^2. What is the energy of the electron in the ground state?

    • A) -13.6 eV
    • B) -6.8 eV
    • C) 0 eV
    • D) 13.6 eV

    Answer: -13.6 eV

    Explanation: For n = 1, E_1 = -13.6 eV; negative energy indicates bound state.

11. Question 11

    Q11. The process by which a nucleus emits an alpha particle is known as

    • A) Alpha decay
    • B) Beta decay
    • C) Gamma decay
    • D) Proton emission

    Answer: Alpha decay

    Explanation: Alpha decay involves emission of 2 protons and 2 neutrons as a single particle.

12. Question 12

    Q12. When a positron and an electron collide, they undergo

    • A) Annihilation
    • B) Pair production
    • C) Compton scattering
    • D) Bremsstrahlung

    Answer: Annihilation

    Explanation: Annihilation converts matter into energy; typically produces two 0.511 MeV gamma photons.

13. Question 13

    Q13. The intensity of X-rays produced in an X-ray tube depends on

    • A) The number of electrons hitting the target
    • B) The energy of the electrons hitting the target
    • C) The wavelength of the X-rays
    • D) The type of target material

    Answer: The number of electrons hitting the target

    Explanation: Intensity is proportional to the number of electrons (tube current), not their energy.

14. Question 14

    Q14. The rest mass energy of an electron is approximately

    • A) 0.511 MeV
    • B) 1.022 MeV
    • C) 931.5 MeV
    • D) 938 MeV

    Answer: 0.511 MeV

    Explanation: 0.511 MeV is the rest mass energy of an electron (or positron); it's a fundamental constant.

15. Question 15

    Q15. The Compton shift in wavelength is given by Δλ = λ' - λ = (h/m_ec)(1 - cosθ). What does 'm_e' represent?

    • A) Mass of the electron
    • B) Mass of the photon
    • C) Mass of the proton
    • D) Mass of the neutron

    Answer: Mass of the electron

    Explanation: m_e is the rest mass of the electron; it's a key parameter in Compton scattering.

16. Question 16

    Q16. The maximum kinetic energy of the photoelectrons emitted from a metal surface is

    • A) Directly proportional to the frequency of the incident light
    • B) Directly proportional to the intensity of the incident light
    • C) Inversely proportional to the work function of the metal
    • D) Independent of the frequency of the incident light

    Answer: Directly proportional to the frequency of the incident light

    Explanation: K_max = hf - φ; it's directly related to frequency, not intensity, and φ is the work function.

17. Question 17

    Q17. What is the term for the minimum frequency of light required to eject electrons from a metal surface?

    • A) Threshold frequency
    • B) Cut-off frequency
    • C) Resonance frequency
    • D) Critical frequency

    Answer: Threshold frequency

    Explanation: Threshold frequency is the minimum frequency required; below this, no electrons are emitted.

18. Question 18

    Q18. The energy released when a nucleus is formed from its constituent nucleons is called

    • A) Binding energy
    • B) Nuclear energy
    • C) Ionization energy
    • D) Excitation energy

    Answer: Binding energy

    Explanation: Binding energy is the energy required to disassemble a nucleus into its nucleons.

19. Question 19

    Q19. In the context of radioactive decay, what is the term for the time required for the activity of a sample to decrease by half?

    • A) Half-life
    • B) Mean lifetime
    • C) Decay constant
    • D) Radioactive period

    Answer: Half-life

    Explanation: Half-life is the time for activity (or number of nuclei) to reduce by half; it's a characteristic property.

20. Question 20

    Q20. The Bremsstrahlung radiation is produced when

    • A) High-energy electrons are rapidly decelerated
    • B) High-energy photons interact with matter
    • C) Alpha particles collide with nuclei
    • D) Beta particles are emitted

    Answer: High-energy electrons are rapidly decelerated

    Explanation: Bremsstrahlung is produced when electrons are decelerated; it's a source of continuous X-ray spectrum.