Dawn of Modern Physics

Dawn of Modern Physics MDCAT MCQs with answers

45 past paper MCQs from Physics unit 14, Dawn of Modern Physics, across 1 topic. They are arranged topic by topic; bigger topics show a sample here and link to their full set and to study notes. Tap “Show answer” for the correct option and a short solution.

Practise as a quiz

Quantum Theory and Photons

1.The process of ejection of loosely bound electrons from a photosensitive surface by the absorption of photons is called: (MDCAT 2022)

  1. the Compton effect
  2. the photoelectric effect
  3. pair production
  4. black body radiation
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Correct answer: (b) the photoelectric effect

One photon gives its whole energy to one electron and ejects it, which is the photoelectric effect.

2.In a photoelectric effect experiment, the stopping potential is: (MDCAT 2022)

  1. the kinetic energy of the most energetic electron ejected
  2. the potential energy of the most energetic electron ejected
  3. the photon energy
  4. the electric potential that causes the electron current to vanish
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Correct answer: (d) the electric potential that causes the electron current to vanish

It is the reverse potential at which even the fastest photoelectron is turned back, so the current falls to zero.

3.The value of Planck's constant is: (MDCAT 2024)

  1. $6.63\times10^{-34}\ \mathrm{J\,s}$
  2. $6.63\times10^{34}\ \mathrm{J\,s}$
  3. $6.63\times10^{-24}\ \mathrm{J\,s}$
  4. $6.63\times10^{-34}\ \mathrm{J\,s^{-1}}$
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Correct answer: (a) $6.63\times10^{-34}\ \mathrm{J\,s}$

$h=6.63\times10^{-34}\ \mathrm{J\,s}$, and $E=h

4.Light propagates in space as a wave, as is evident by all of the following EXCEPT: (MDCAT 2024)

  1. interference
  2. photoelectric effect
  3. diffraction
  4. polarization
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Correct answer: (b) photoelectric effect

The photoelectric effect is the evidence for the particle model of light, not for the wave model.

5.If light whose photons carry an energy of \$2.4\,\mathrm{eV}\$ is used, the stopping potential required will be: (UHS MDCAT 2017)

  1. $2.4\,\mathrm{V}$
  2. $2.6\,\mathrm{V}$
  3. $2.5\,\mathrm{V}$
  4. $0\,\mathrm{V}$
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Correct answer: (a) $2.4\,\mathrm{V}$

Taking the whole photon energy as the maximum kinetic energy, $eV_o=2.4\,\mathrm{eV}$, so $V_o=2.4\,\mathrm{V}$.

6.The de Broglie wavelength of a particle of mass \$1\,\mathrm{g}\$ moving with a velocity of \$100\,\mathrm{m\,s^{-1}}\$ is: (UHS MDCAT 2017)

  1. $6.63 imes10^{-35}\,\mathrm{m}$
  2. $6.63 imes10^{-34}\,\mathrm{m}$
  3. $6.63 imes10^{-33}\,\mathrm{m}$
  4. $6.63 imes10^{-32}\,\mathrm{m}$
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Correct answer: (c) $6.63 imes10^{-33}\,\mathrm{m}$

$\lambda=\dfrac{h}{mv}=\dfrac{6.63\times10^{-34}}{10^{-3}\times100}=6.63\times10^{-33}\,\mathrm{m}$.

7.The energy of a quantum is given by which equation? (KMU MDCAT 2025)

  1. E = mc^2
  2. E = hf
  3. E = 1/2 mv^2
  4. E = qV
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Correct answer: (b) E = hf

Planck’s relation gives the energy of one quantum as $E=hf$.

8.In photoelectric effect, stopping potential depends upon: (NUMS MDCAT 2025)

  1. Intensity of light
  2. Frequency of light
  3. Speed of light
  4. Amplitude of light
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Correct answer: (b) Frequency of light

The stopping potential measures the maximum kinetic energy, which depends on the frequency alone.

9.If different colored light beams have same total energy, which color beam will contain the smallest number of photons? (SZABMU MDCAT 2025)

  1. Violet
  2. Blue
  3. Green
  4. Red
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Correct answer: (a) Violet

Violet photons carry the most energy each, so the fewest of them are needed to make up the same total.

10.According to the particle model of light, a photon is: (SZABMU MDCAT 2025)

  1. A particle with mass and charge
  2. A quantum of energy with zero rest mass and zero charge
  3. A continuous energy wave
  4. A particle that travels slower than light
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Correct answer: (b) A quantum of energy with zero rest mass and zero charge

A photon is a packet of energy with no rest mass and no charge.

11.The value of Planck constant is (UHS MDCAT 2024)

  1. 6.63x10^-34 Js
  2. 6.63x10^34 Js
  3. 6.63x10-34 Js-1
  4. 6.63x10^34 Js-1
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Correct answer: (a) 6.63x10^-34 Js

Planck’s constant is $6.63\times10^{-34}\,\mathrm{J\,s}$.

12.The de-Broglie wavelength associated with a particle moving at 10^6 m/s and having mass 10^-30 kg (UHS MDCAT 2024)

  1. 6.6x10^-10 m
  2. 1.5x10^9 m
  3. 1.9x10^-5 m
  4. 7.2x10^-8 m
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Correct answer: (a) 6.6x10^-10 m

$\lambda=h/mv=6.63\times10^{-34}/(10^{-30}\times10^{6})$, which is $6.6\times10^{-10}\,\mathrm{m}$.

13.Light propagates through space as a wave is evident by all of the following EXCEPT (UHS MDCAT 2024)

  1. Interference
  2. Photoelectric effect
  3. Diffraction
  4. Polarization
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Correct answer: (b) Photoelectric effect

Interference, diffraction and polarization are wave effects; the photoelectric effect is what showed light to be particulate.

14.The momentum of moving photon is: (KMU MDCAT 2024)

  1. mc^2
  2. λ / h
  3. h / λ
  4. zero
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Correct answer: (c) h / λ

A photon has no mass, but it carries momentum $h/\lambda$.

15.In every instant of time, wavelength associated with a freely falling body: (KMU MDCAT 2024)

  1. Decreases
  2. Increases two times
  3. Increases four times
  4. Remains constant
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Correct answer: (a) Decreases

It falls faster and faster, and $\lambda=h/mv$ shrinks as the speed grows.

16.As per 2nd photoelectric experiment, photoelectric effect does not occur if the frequency of the incident light is? (KMU MDCAT 2024)

  1. Below the threshold frequency
  2. Equals the threshold frequency
  3. Three times the threshold frequency
  4. Twice the threshold frequency
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Correct answer: (a) Below the threshold frequency

Below the threshold no photon carries enough energy to free an electron, however bright the light.

17.Q.169 If electron, proton, neutron, and alpha particle have same velocity, which of them has the shortest wavelength? (UHS MDCAT 2023)

  1. Electron
  2. Proton
  3. Neutron
  4. Alpha particle
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Correct answer: (d) Alpha particle

At the same speed the alpha particle has much the largest momentum, and $\lambda=h/mv$ is smallest for it.

18.Q.170 The process of ejection of loosely bound electrons from a certain photo sensitive surface by absorption of photon is called: (UHS MDCAT 2023)

  1. Compton effect
  2. Photoelectric effect
  3. Pair production
  4. Black body radiation
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Correct answer: (b) Photoelectric effect

That is the photoelectric effect.

19.Q.171 In a photoelectric effect experiment, the stopping potential is: (UHS MDCAT 2023)

  1. The kinetic energy of the most energetic electron ejected
  2. The potential energy of the most energetic electron ejected
  3. The photon energy
  4. The electric potential that causes the electron current to vanish
Show answer

Correct answer: (d) The electric potential that causes the electron current to vanish

The stopping potential is the reverse voltage at which even the fastest electron fails to reach the anode.

20.Which photons carries the most energy? (SZABMU MDCAT 2023)

  1. Blue
  2. Violet
  3. Red
  4. Green
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Correct answer: (b) Violet

Energy is $hf$, and violet has the highest frequency of the four.

21.Wave nature and particle nature of photon is linked by: (PMC MDCAT 2022)

  1. Rest mass of photon
  2. Wavelength of photon
  3. Light speed
  4. Momentum of photon
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Correct answer: (d) Momentum of photon

$p=h/\lambda$ ties the momentum of the particle to the wavelength of the wave.

22.The word radiation means: (PMC MDCAT 2022)

  1. To propagate
  2. stay at rest
  3. Mass less objects
  4. Charged object
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Correct answer: (a) To propagate

To radiate is to send energy out through space.

23.If the temperature of Sun is doubled, heat energy reached on earth become: (PMC MDCAT 2022)

  1. Three Times
  2. Sixteen Times
  3. Four Times
  4. Same
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Correct answer: (b) Sixteen Times

By Stefan’s law the power radiated goes as $T^4$, so twice the temperature is sixteen times the energy.

24.A body at temperature T radiates heat according to relation: (PMC MDCAT 2022)

  1. T2
  2. T4
  3. T-4
  4. T-3
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Correct answer: (b) T4

Stefan’s law gives the radiated power as proportional to the fourth power of the temperature.

25.Photons of energy 10.25 eV fall on the surface of the metal emitting photoelectrons of maximum kinetic energy 5.0 eV. What is the stopping voltage required for these electrons? (PMC MDCAT 2022)

  1. 10V
  2. 8V
  3. 5V
  4. 4V
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Correct answer: (c) 5V

The stopping potential is the maximum kinetic energy in electron volts, that is $5\,\mathrm{V}$.

26.Blackbody shows a ______ spectra: (PMC MDCAT 2022)

  1. Continuous
  2. Discrete
  3. Band
  4. Line
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Correct answer: (a) Continuous

A hot body radiates at every wavelength, so its spectrum is continuous.

27.Planck’s constant is analogous to: (PMC MDCAT 2022)

  1. Inertia
  2. Wave nature
  3. Angular momentum
  4. Linear momentum
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Correct answer: (c) Angular momentum

Planck’s constant has the units of energy times time, which are the units of angular momentum.

28.Among the following four spectral regions, which photon has the highest energy? (PMC MDCAT 2022)

  1. Infrared
  2. Violet
  3. Blue
  4. red
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Correct answer: (b) Violet

Energy is $hf$, and violet has the highest frequency of the four.

29.Planck constant is named after: (PMC MDCAT 2022)

  1. Einstein
  2. Newton
  3. Maxwell
  4. Max planck
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Correct answer: (d) Max planck

It is named after Max Planck, who introduced it in 1900.

30.The black body always __ radiations: (PMC MDCAT 2022)

  1. Emit
  2. Absorb
  3. Both Emit And Absorb
  4. Reflects
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Correct answer: (c) Both Emit And Absorb

A black body is a perfect absorber and, at the same temperature, a perfect emitter.

31.Photocell is based on: (PMC MDCAT 2022)

  1. Photoelectric effect
  2. Compton effect
  3. Photoluminescence
  4. Pair production
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Correct answer: (a) Photoelectric effect

Light falling on the cathode of a photocell frees electrons, which is the photoelectric effect.

32.Work function depends on: (PMC Practice 2021)

  1. Structure of metals only
  2. Nature of surface only
  3. Both metals and nature of surface
  4. Threshold frequency
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Correct answer: (c) Both metals and nature of surface

The work function is the energy needed to free an electron, and that depends on the metal and on the state of its surface.

33.Momentum of a photon is? (PMC Practice 2021)

  1. 0
  2. λ
  3. hf/c
  4. mv
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Correct answer: (c) hf/c

A photon carries momentum $E/c$, that is $hf/c$.

34.What is the de Broglie wavelength associated with an electron, accelerated through a potential difference of 200 volts? (PMC Practice 2021)

  1. 1 nm
  2. 0.5 nm
  3. 0.0056 nm
  4. 0.086 nm
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Correct answer: (d) 0.086 nm

$\lambda=12.27/\sqrt{V}$ ångström, and $\sqrt{200}$ gives about $0.087\,\mathrm{nm}$.

35.The wavelength associated with an electron is of the order of: (PMC MDCAT 2020)

  1. Visible light
  2. X rays
  3. Radio waves
  4. Infrared
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Correct answer: (b) X rays

An accelerated electron has a wavelength of about an ångström, the wavelength of X-rays.

36.Which photon carries the most energy? (PMC MDCAT 2020)

  1. Blue
  2. Violet
  3. Red
  4. Green
Show answer

Correct answer: (b) Violet

Energy is $hf$, and violet has the highest frequency of the four.

37.The value and units of the Planck constant ‘h’ can be expressed as: (UHS MDCAT 2019)

  1. 3.63 x 10-34 Js
  2. 6.63 x 10-34 Js-1
  3. 6.63 x 10-34 Js
  4. 6.63 x 10-43 Js
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Correct answer: (c) 6.63 x 10-34 Js

Planck’s constant is $6.63\times10^{-34}\,\mathrm{J\,s}$.

38.Minimum energy required to eject an electron from metal surface is called: (UHS MDCAT 2019)

  1. Stopping potential
  2. Electromotive force
  3. Work function
  4. Threshold frequency
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Correct answer: (c) Work function

That least energy is the work function of the metal.

39.Calculate the energy of a photon, with a frequency of 3.0 x 1018 Hz. (UHS MDCAT 2019)

  1. 19.89 x 10-16 J
  2. 11.89 x 10-16 J
  3. 1.89 x 10-16 J
  4. 19.89 x 10-18 J
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Correct answer: (a) 19.89 x 10-16 J

$E=hf=6.63\times10^{-34}\times3.0\times10^{18}$, that is $19.89\times10^{-16}\,\mathrm{J}$.

40.A 5 watt LED bulb converts 80% of the power into light photons wavelength 660 nm. What is the number of photons emitted from the bulb in one second. (UHS MDCAT 2018)

  1. 5.8 x 1034
  2. 7.5 x 1018
  3. 6.6 x 107
  4. 1.3 x 1019
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Correct answer: (d) 1.3 x 1019

$4\,\mathrm{W}$ of light with photons of $hc/\lambda=3.0\times10^{-19}\,\mathrm{J}$ gives about $1.3\times10^{19}$ photons a second.

41.Light photons, each of energy 3.5 x 10-19J falls on the cathode of a photocell. The current through the cell is reduced to zero by taking the cathode to a potential +0.25 V relative to anode. The work function of the cathode is: (UHS MDCAT 2018)

  1. 3.35 x 10-19J
  2. 3.5 x 10-19J
  3. 3.25 x 10-19J
  4. 3.1 x 10-19J
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Correct answer: (d) 3.1 x 10-19J

$W=hf-eV_s=3.5\times10^{-19}-1.6\times10^{-19}\times0.25$, that is $3.1\times10^{-19}\,\mathrm{J}$.

42.The de Broglie wavelength of an electron travelling with a speed of 1.0 x 107m/s is equal to, (h=6.6 x 10-34 Js and me= 9.1 x 10-31 kg) (UHS MDCAT 2018)

  1. 7.3 x 1011m
  2. 7.3 x 108m
  3. 7.3 x 10-11m
  4. 7.3 x 10-13m
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Correct answer: (c) 7.3 x 10-11m

$\lambda=h/mv=6.6\times10^{-34}/(9.1\times10^{-31}\times10^{7})$, about $7.3\times10^{-11}\,\mathrm{m}$.

43.If we use a light of photons having energy 2.4 eV then the stopping potential required will be: (UHS MDCAT 2017)

  1. 2.4 V
  2. 2.6 V
  3. 2.5 V
  4. 0 V
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Correct answer: (a) 2.4 V

With no work function to subtract the stopping potential would be the full $2.4\,\mathrm{V}$.

44.The de Broglie wavelength of a particle of mass 1 gram and velocity 100 ms-1 is: (UHS MDCAT 2017)

  1. 6.63 x 10-35
  2. 6.63 x 10-34
  3. 6.63 x 10-33
  4. 6.63 x 10-32
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Correct answer: (c) 6.63 x 10-33

$\lambda=h/mv=6.63\times10^{-34}/(10^{-3}\times100)$, that is $6.63\times10^{-33}\,\mathrm{m}$.

45.When light has exactly the same energy as the work done function of a metal surface then: (UHS MDCAT 2017)

  1. No photoelectrons are produced
  2. No photoelectric effect is observed
  3. No photo electric current is produced
  4. All of these options are correct
Show answer

Correct answer: (c) No photo electric current is produced

The electrons are just freed with no energy to spare, so none reaches the anode and no current flows.

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