12th Class Physics MCQs Preparation 2022 and Onwards (Unit: Dawn of Modern Physics : Set-2 (23 MCQs)) for National MDCAT, ECAT, FPSC, PPSC, JOB Exam, Physics Lecturer, SSC, HSSC (F.Sc.), BS, MS Exams NTS, ETS, etc..



1. Triplets A and B board spaceships and head away from Earth in opposite directions, each traveling the same distance at the same speed before returning to Earth. Triplet C remains on Earth. Which expression describes the triplets' relative ages once they're reunited on Earth?




... Answer is C. A = B ＜ C
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2. If a spaceship travels from Earth to a planet in the Alpha Centauri star system, which observer measures the proper time for the trip?




... Answer is C. an observer on the spaceship
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3. When you compress a spring between your fingers, its mass




... Answer is A. Increase
A compressed spring has more mass than an uncompressed spring


4. By doubling the absolute temperature of the black body, the total energy radiated will change by a factor of




... Answer is D. 16
According to Stefan-Boltzmann Law, E = σ T⁴


5.  According to Einstein, increasing the brightness of a beam of light without changing its color will increase:




... Answer is A. the number of photons.
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6. Sources of red, blue, and yellow light each emit light with a power of 50 mW. The source which emits more photons is




... Answer is A. red
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7. Of the following energies for photons, which is the least energy that could result in photoelectron production if the work function is 3.00 eV? :




... Answer is C. 3.50 eV
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8. In a Compton scattering experiment at which angle does the electron carry maximum energy? (Assume that initially, the target electron is at rest).




... Answer is B. 90°
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9. If X-rays are scattered from protons instead of electrons, the change in its wavelength for a given angle of scattering




... Answer is B. decreases
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10. Pair production cannot take place in a vacuum because.




... Answer is B. Momentum is not conserved
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11. According to de-Broglie equation which one has the smallest wavelength associated with it




... Answer is C. α-particle
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12. An electron and a proton have the same de-Broglie wavelength. Then the kinetic energy of the electron is




... Answer is D. greater than the kinetic energy of the proton
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13. In an Electron microscope, the light source is replaced by a beam of very fast moving




... Answer is A. electron
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14.  According to Heisenberg, as the uncertainty in the measurement of a particle's momentum is reduced by a factor of 2, by what factor is the uncertainty in that same particle's position changed?




... Answer is c. 2
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15. If the length measured by the rest observer is `\l_0` and the length measured by the moving observer is l, then the relation for length contrition in relativistic mechanics is




... Answer is A. l = `\l_0 sqrt {1-v^2/c^2}`
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. l = `\l_0 sqrt {1-v^2/c^2}`


16.. If you were traveling in a spaceship at a speed of c/2 relative to the earth and you fires a laser beam in the direction of the spaceship's motion. What will be the light speed from the laser relative to earth?




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


17. Let two observers A and B measure the speed of light in a vacuum, such that on observer A is at rest with respect to the source of light and observer B is moving away from it with uniform velocity. According to Einstein's principle of relativity, the result of both observers will be the: :




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


18. The special theory of relativity is based on how many postulate ?




... Answer is B. 2
The special theory of relativity is based on two postulates. (1) All laws of physics are the same in all inertial frames of reference. (2) The speed of light is a universal constant. That is, the values of the speed of light (3 ｘ10⁸ m/s) remain the same in all inertial frames of reference.


19. The speed of light is a universal constant. Its values remain the same in all inertial frame of reference. The value of the speed of light c is:




... Answer is B. c = 3 ｘ10⁸ m/s
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20. Special theory was proposed in 1905 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. A frame of reference which is at rest or moving with uniform velocity is called




... Answer is A. Inertial frame of reference
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23. Let you be standing on the roof of the building and you are observed by two observers one is on the moon and the other is at the geostationary satellite. Who will observe you that you are in motion?




... Answer is A. Observer at the moon
You will appear at rest (not moving with earth's motion) with respect to the geostationary satellite, while appearing moving in motion with respect to the moon.






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