12th Class Physics MCQs Preparation 2022 and Onwards (Unit: Dawn of Modern Physics : Set-1 (30 MCQs)) for National MDCAT, ECAT, FPSC, PPSC, JOB Exam, Physics Lecturer, SSC, HSSC (F.Sc.), BS, MS Exams NTS, ETS, etc..



1. Inertial frame of reference is one that satisfies




... Answer is A. Newton's Theory
In an inertial frame of reference, the acceleration of the body is zero, and the law of inertia remains valid in it. That is, if a body is at rest it will remain at rest and if a body is moving with a uniform velocity it will continue its uniform motion unless an unbalanced force produces an acceleration in it.


2. Non-inertial frame of reference has




... Answer is D. Variable velocity
A frame of reference that is accelerating (variable velocity), or decelerating is known as a Non-inertial frame of reference.


3. If the source of light is moving toward the observer, then the speed of light received by the observer will be




... Answer is C. Remain same
According to the 2nd postulate of the Special theory of relativity, the speed of light (3 ｘ10⁸ m/s) is constant in all inertial frames of reference.


4. The relativistic length of a moving object will be




... Answer is B. Decreased
According to Einstein's special theory of relativity, the length of an object measured in a frame of reference which is moving with respect to the object is always less than the rest length or proper length. This effect is known as length contraction.


5.  If the rest mass of a particle is zero, then its speed is




... Answer is A. Equal to speed light
m = `\frac {m_0}{sqrt {1-v^2/c^2}}` when `\m_0` = 0 then v = c (after simplifying the relation). eg. the rest mass of photon in free space is zero and moves with the velocity of light.


6. Blackbody radiation depends upon




... Answer is C. Temperature
Stefan-Boltzmann Law, E = σ T⁴


7. If the temperature of black body is doubled then the emitted energy from it will be increased :




... Answer is D. Sixteen time
Stefan-Boltzmann Law, E = σ T⁴


8. The dimension of Planck's constant is




... Answer is B. [ML²T⁻¹]
h = `\frac {E}{f}`-----(1), Dimensions of [E] = [ML²T⁻²], Dimensions of frequency [f]=[ T⁻¹] thus (1) h = `\frac {[ML²T⁻²]}{[ T⁻¹]}` = [ML²T⁻¹]


9. Who did observe 1st time the photoelectric effect?




... Answer is B. Hertz
Heinrich Hertz in 1887


10. Photo electric effect depends upon the photon's




... Answer is D. Frequency
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11. A photon can transfer its energy into an electron, it was first explained by




... Answer is C. Einstein
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12. The momentum of a photon is




... Answer is D. `\frac {hf}{c}`
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13. A change in energy of a photon occurs when it collides with an electron at rest and is known as




    ... Answer is B. Compton effect
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    14.  Which phenomenon does not verify the particle nature of light




    ... Answer is D. diffraction
    In Diffraction, light bends at the sharp ends of an obstacle or a hole. So, it also cannot verify the particle's nature of light.


    15. The antiparticle of electron is




    ... Answer is D. Positron
    Positron is the antiparticle of electron. Both have the same rest mass but opposite charges.


    16.. Davisson and Germer proved experimentally the wave nature of particles under the phenomenon of




    ... Answer is D. Diffraction
    In Diffraction, light bends at the sharp ends of an obstacle or a hole. So, it also cannot verify the particle's nature of light.


    17. In the electron-positron pair production, the speed of the electron is :




    ... Answer is C. Equals to speed of positron
    Positron is the antiparticle of electron. Both have the same rest mass and speed but opposite charges.


    18. If the energy of the used light is high, then the momentum of the investigated particle has




    ... Answer is B. High uncertainty
    Light of shorter wavelengths has high energy. Light of a shorter wavelength disturbs the momentum of a particle, as a result, there is more uncertainty in the measurement of the momentum of a particle. (Δp ≈ `\frac {h}{λ}`)


    19. For small uncertainty in the measurement of the position of a particle, the wavelength of the incident light should be




    ... Answer is A. Small
    To reduce the uncertainty in the position of a particle, we must use light of shorter wavelength because Δx ≈ λ


    20. According to quantum theory, light consists of a small packet of energy. This packet of energy was named a "Photon" by




    ... Answer is C. Einstein
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    21. The value of Stefan's constant is




    ... Answer is C. 5.67 ｘ 10⁻⁸ w m⁻² K⁻⁴
    Stefan's constant σ = `\frac {E}{T^4}` = 5.67 ｘ 10⁻⁸ w m⁻² K⁻⁴, where E is the Energy per second per unit area (total radiant Power). (A) Wien's constant = 2.9 ｘ 10⁻³ m K, (B) Boltzmann's constant = 1.3807 ｘ 10⁻²³ J K, (C) Plank's Constant = 6.67 ｘ 10⁻²³ J s


    22. The dimension of Stefan's constant is




    ... Answer is A. [M⁰L⁰T⁻³K⁻⁴]
    σ = `\frac {E}{T^4}`-----(1), Dimensions of Energy per second per unit area is [E] = [M⁰L⁰T⁻³], Dimensions of Temperature [T]=[ K] thus (1) h = `\frac {[M⁰L⁰T⁻³]}{[ K^4]}` = [M⁰L⁰T⁻³K⁻⁴


    23. The sequence of colour changed by the blackbody on increasing the temperature is:




    ... Answer is D. Red, Yellow, White
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    24. If the moving observer is also at rest (ie. v = 0 m/s) from the relation t = `\frac {t_0}{sqrt {1-v^2/c^2}}` we conclude:




    ... Answer is A. t = `\t_0`
    t = `\frac {t_0}{sqrt {1-v^2/c^2}}` when v = 0 m/s then the denominator becomes equal to 1 thus t = `\t_0`


    25.  The relation between the observed time t and the proper `\t_0` in relativistic mechanics is




    ... Answer is C. t > `\t_0`
    According to Einstein's special theory of relativity, the time dilation equation t = `\frac {t_0}{sqrt {1-v^2/c^2}}`, the denominator will always be less than 1 thus, the observed time t of an event is always greater than the proper time `\t_0` (i.e. t > `\t_0`)



26. Two identical constructed clocks are synchronized. One is attached to the wall of moving spaceships while the other remains on the earth. Which clock runs faster?




... Answer is A. On the earth
As the clock in the spaceship will run slowly due to time dilation (become Larger). So the clock on the earth will appear to run fast as compared to the clock on a spaceship.


27. If the time measured by the observer at rest frame is `\t_0` (Proper Time) and the time measured by the moving observer moving with uniform velocity v is t, then the relation for time dilation in relativistic mechanics is :




... Answer is B. t = `\frac {t_0}{sqrt {1-v^2/c^2}}`
According to Einstein's special theory of relativity, the time of an event measured in a frame of reference which is moving with respect to the object is always greater than the rest time or proper time. This effect is known as length dilation (ie. becomes larger). t = `\frac {t_0}{sqrt {1-v^2/c^2}}`


28. If a rod of length 1m moves with a very high speed, will you observe its length




... Answer is B. l < `\l_0`
According to Einstein's special theory of relativity, the length contraction. `\l` = `\l_0 sqrt {1-v^2/c^2}`, the observed length `\l` is always less than the proper length l (i.e. `\l` < `\l_0`)


29. If the moving observer is also at rest (ie. v = 0 m/s) from the relation l = `\l_0 sqrt {1-v^2/c^2}` we conclude:




... Answer is A. `\l` = `\l_0`
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30. If the length measured by the rest observer is `\l_0` and the length measured by the moving observer is l, then according to the relativistic mechanics:




... Answer is A. `\l` < `\l_0`
According to Einstein's special theory of relativity, the length contraction. `\l` = `\l_0 sqrt {1-v^2/c^2}`, So `\l` < `\l_0`






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