# Modern Physics-04- Objective Unsolved Level

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26 May 2023
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### Modern Physics-04- Objective Unsolved Level

• 1. O OB BJ JE EC CT TI IV VE E U UN NS SO OL LV VE ED D L LE EV VE EL L - - I I 1. In which of the following transitions will the wavelength be minimum? (a) 5 n  to 4 n  (b) 4 n  to 3 n  (c) 3 n  to 2 n  (d) 2 n  to 1 n  . 2. In which of the following systems will the radius of the first orbit ( 1) n  be a minimum? (a) hydrogen atom (b) deuterium atom (c) singly ionized helium (d) doubly ionized lithium. 3. Which of the following curves may represent the speed of the electron in a hydrogen atom as a function of the principal quantum number n ? a b c d v n 4. Which of the following parameters are the same for all hydrogen-like atoms and ions in their ground states? (a) radius of the orbit (b) speed of the electron (c) energy of atom (d) orbital angular momentum of the electron. 5. Frequencies of K X-rays of different materials are measured. Which one of the graphs in figure may represent the relation between the frequency v and the atomic number Z ? v d c b a z 6. X-rays incident on a material (a) exerts a force on it (b) transfers energy to it (c) transfers momentum to it (d) transfers impulse to it. 7. The maximum kinetic energy of photoelectrons emitted from a surface when photons of energy 6 eV fall on it is 4 eV. The stopping potential in volts is : (a) 2 (b) 4 (c) 6 (d) 10. 8. According to the Bohr model, the minimum energy (in eV) required to remove an electron from the ground state of doubly ionized Li atom (Z = 3) is : (a) 1.51 (b) 13.6 (c) 40.8 (d) 122.4. 9. The half life of 131 I is 8 days. Given a sample of 131 I at time t = 0, we can assert that (a) No nucleus will decay before t = 4 days (b) No nucleus will decay before t = 8 days (c) All nucleus will decay before t = 16 days (d) A given nucleus may decay at any time after t = 0.
• 2. 10. The work function of a substance is 4.0 eV. The longest wavelength of light that can cause photoelectron emission from this substance is approximately : (a) 540 nm (b) 400 nm (c) 310 nm (d) 220 nm. 11. Electrons with energy 80 keV are incident on the tungsten target of an X-Ray tube. K-shell electrons of tungsten have -72.5 keV energy. X-rays emitted by the tube contain only (a) A continuous X-ray spectrum (Bremsstrashlung) with a minimum wavelength of ~ 0.155 A. (b) A continuous X-ray spectrum (Bremsstrahlung) with all wavelengths. (c) The characteristic X-ray spectrum of tungsten. (d) A continuous X-ray spectrum (Bremsstrahlung) with a minimum wavelength of ~ 0.155 A and the characteristic X-ray spectrum of tungsten. 12. Two radioactive materials 1 X and 2 X have decay constants 10  and  respectively. If initially they have the same number of nuclei, then the ratio of the number of nuclei of 1 X to that of 2 X will be 1/e after a time (a) 1/10  (b) 1/11  (c) 11/10  (d) 1/9  . 13. The electron in a hydrogen atom makes a transition from an excited state to the ground state. Which of the following statements is true (a) Its kinetic energy increases and, its potential and total energy decrease. (b) Its kinetic energy decreases, potential energy increases and its total energy remains the same. (c) Its kinetic and total energy decreases and, its potential energy increases. (d) Its kinetic, potential and total energy decreases. 14. A sample of radioactive material has mass m , decay constant  , and molecular weight M . Avogadro constant A N  . The initial activity of the sample is (a) m  (b) m M  (c) A m N M  (d) A mN e . 15. If a potential difference of 20,000 volts is applied across an X-ray tube, the cut-off wavelength will be (a) 10 6.21 10  m (b) 11 6.21 10  m (c) 12 6.21 10  m (d) 11 3.1 10  m. O OB BJ JE EC CT TI IV VE E U UN NS SO OL LV VE ED D L LE EV VE EL L - - I II I 1. The transition from the state n = 4 to n = 3 in a hydrogen like atom results in ultraviolet radiation. Infrared radiation will be obtained in the transition : (a) 2 1  (b) 3 2  (c) 4 3  (d) 5 4  . 2. X-rays are produced in an X-ray tube operating at a given accelerating voltage. The wavelength of the continuous X-rays has values from : (a) 0 to  (b) min min to where 0     (c) max max 0 to where    (d) min max min max to where 0       .
• 3. 3. The intensity of X-Rays from a Coolidge tube is plotted against wavelength  as shown in the figure. The minimum wavelength found is C  and the wavelength of the k line is K  . As the accelerating voltage is increased : (a) K C    increases (b) k c    decreases (c) k  increases (d) k  decreases. I  k c 4. A radioactive sample consists of two distinct species having equal number of atoms initially. The mean life of one species is  and that of the other is 5  . The decay products in both cases are stable. A plot is made of the total number of radioactive nuclei as a function of time. Which of the following figures best represents the form of this plot : (a) N t  (b) N t  (c) N t  (d) N t  . 5. The potential difference applied to an X-ray tube is 5 kV and the current through it is 3.2 mA. Then the number of electrons striking the target per second is (a) 16 2 10  (b) 6 5 10  (c) 17 1 10  (d) 15 4 10  . 6. A hydrogen atom and a ++ Li ion are both in the second excited state. If and H Li I I are their respective electronic angular momenta, and and H Li E E their respective energies then (a) and | | | | H Li H Li I I E E   (b) and | | | | H Li H Li I I E E   (c) and | | | | H Li H Li I I E E   (d) and | | | | H Li H Li I I E E   . 7. The half – life of 215 At is100 s  . The time taken for the radioactivity of a sample of 215 At to decay to 1/16th of its initial value is : (a) 400 s  (b) 63 s  (c) 40 s  (d) 300 s  . 8. Which of the following processes represents  - decay ? (a) 1 A A Z Z X X a b       (b) 1 3 0 2 A A Z Z X n X c      (c) A A Z Z X X f   (d) 1 1 A A Z A X e X g      . 9. The electric potential between a proton and an electron is given by 0 0 In r V V r  , 0 r is a constant. Assuming Bohr’s model to be applicable, write variation of n r with n, n being the principal quantum number ? (a) n r n  (b) 1 n r n  (c) 2 n r n  (d) 2 1 n r n  .
• 4. 10. If the atom 257 100 Fm follows the Bohr model and the radius of 257 100 Fm is n times the Bohr radius, then find n . (a) 100 (b) 200 (c) 4 (d) ¼. 11. For uranium nucleus how does its mass vary with volume ? (a) m V  (b) 1/ m V  (c) m V  (d) 2 m V  . 12. A nucleus with mass number 220 initially at rest emits an  - particle. If the Q value of the reaction is 5.5 Mev, calculate the kinetic energy of the  - particle: (a) 4.4 MeV (b) 5.4 MeV (c) 5.6 MeV (d) 6.5 MeV. 13. After 280 days, the activity of a radioactive sample is 6000 dps. The activity reduces to 3000 dps after another 140 days. The initial activity of the sample in dps is : (a) 6000 (b) 9000 (c) 3000 (d) 24000. 14. The energy of a photon is equal to the kinetic energy of a proton. The energy of the photon is E Let 1  be the de-Broglie wavelength of the proton and 2  be the wavelength of the photon. The ratio 1 2   is proportional to (a) Eº (b) E1/2 (c) 1 E (d) 2 E . 15. The K X-ray emission line of tungsten occurs at 0.021   nm. The energy difference between K and L levels in this atom is about (a) 0.51 MeV (b) 1.2 MeV (c) 59 keV (d) 13.6 eV.