CBSE Class 12 Physics Modern Physics Objective Questions

CBSE Class 12 Physics Modern Physics Objective Questions. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

 OBJECTIVES

1. The ratio of momenta of an electron of mass me and an alpha particle of mas mα which are accelerated from rest by a potential difference of 250 volt is...
(a) me/mα  (b) √me/mα (c)  √2me/mα (d)  √me/2mα

2. A potential difference of 20 kV is applied to the anti-cathode of an X-ray tube to attract electrons. The maximum speed of electrons is...
(a) 8.4 × 106 ms^–1 (b) 8.4 × 10⁷ ms^–1 (c) 5.95 × 10⁶ ms^–1 (d) 5.95 × 10⁷ ms^–1

3. A proton and an alpha particle have kinetic energies in the ratio 16 : 1. Assuming that mass of alpha particle is equal to four times the mass of a proton, the ratio of de Broglie’s wavelength of proton to that of alpha particle is...
(a) 1 : 4 (b) 1 : 2 (c) 2 : 1 (d) 4 : 1

4. The radius of a nucleus with mass number 16 is 3.0 Fermi. The radius of another nucleus with mass number 128 is
(a) 6 Fermi (b) 12 Fermi (c) 18 Fermi (d) 24 Fermi

5. The end product of decay of ₉₀Th²³² is ₈₂Pb²⁰⁸. The number of alpha and beta particles emitted are
(a) 4, 6 (b) 6, 4 (c) 6, 0 (c) 6, 3

6. Two radioactive substanes X and Y initially contain equal number of nuclei. X has a half life of 1 hour and Y has a half life of 2 hour. After two hours, the activity of X to that of Y is
(a) 1 : 1 (b) 1 : 2 (c) 2 : 1 (d) 1 : 4

7. The half life of radium is 1620 years and its atomic weight is 226 kg per kilomole. The number of atoms that will decay from its 1 gram sample per second will be
(a) 3.61 × 10¹⁰ (b) 3.61 × 10¹² (c) 3.6 × 10¹⁴ (d) 3.61 × 10¹⁶

8. Masses of two isobars ₂₉Cu^₆₄ and ₃₀Zn⁶⁴ are 63.9298 units and 63.9292 units respectively. It can be concluded from this data that
(a) both isobars are stable
(b) Zn⁶⁴ is radioactive decaying to Cu⁶⁴ through β− -decay
(c) Cu⁶⁴ is radioactive decaying to Zn⁶⁴ through γ-decay
(d) Cu⁶⁴ is radioactive decaying to Zn⁶⁴ through β−-decay

9. A sample contains 16 gm of a radioactive material, the half life of which is 2 days. After 32 days, the amount of radioactive material left in the sample is
(a) less than 1 mg (b) 1/4 gm (c) 1/2 gm (d) 1 gm

10. One gm of a radioactive material having a half life of 2 years is kept in a store for a duration of 4 years.The fraction of material which has decayed is
(a) 1/8 (b) 1/ 4 (c) 1/2 (d) 3/4

11. The mass of 2.0 curie of U234 radioisotope is
(a) 7.4 × 10^–10 gm (b) 1.438 × 10^–11 gm (c) 2.88 × 10^–11 gm (d) 3.7 × 10^–10 gm

12. A radioactive material has a mean life of 1620 years and 540 years for α and β emission respectively. The time in which one half of the material remains undecayed emitting both these radiations is
(a) 202.5 yrs (b) 280.7 yrs (c) 405 yrs (d) 1080 yrs

13. Consider the spectral line resulting from the transition n = 2 → n = 1 in the atoms and ions given below. The shortest wavelength is produce by
(a) hydrogen atom (b) deutrium atom (c) singly ionized helium (d) doubly ionized lithium

14. The ratio of the times taken by the electron in a hydrogen atom to complete one revolution in orbits corresponding to ground and first excited states is
(a) 1 : 4 (b) 4 : 1 (c) 1 : 8 (d) 8 : 1

15. The ratio of the longest and shortest wavelengths of the Lyman series is approximately
(a) 4/3 (b) 9/4 (c) 9/5 (d) 16/7

16. The longest wavelength of the Lyman series is approximately
(a) 920 Å (b) 1020 Å (c) 1220 Å (d) 1420 Å

17. The first excitation potential of an atom is 10.2 V. Its ionization potential is
(a) 40.8 V (b) 30.6 V (c) 20.4 V (d) 13.6 V

18. Energy levels A, B, C of a certain atom correspond to increasing values of energy, i.e., E_A < E_B < E_C. If λ₁, λ₁, λ₃ are the wavelength of radiation corresponding to the transitions C → B, B → A and C → Arespectively, then

(a) λ₃ = λ₁ + λ₂ (b) λ₃ =λ₁λ₂/λ ₁+λ₂ (c) λ^2/3 = λ^2/1 + λ ^2/2  (d) λ₁ = λ₂ + λ₃

19. An electron jumps from the fourth to the second orbit of hydrogen atom. Given the Rydberg constant R = 10⁷ m^–1, the frequency of the emitted radiation, in Hz, is
(a) (3/16) 10¹⁵ (b) (3/4) 10¹⁵ (c) (9/4) 10¹⁵ (d) (9/16) 10¹⁵

20. The first spectral line of sodium is 5890 Å. The first excitation potential of sodium is
(a) 2.1 V (b) 3.7 V (c) 4.1 V (d) 7.4 V

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