#### 12th Standard Physics English Medium Dual Nature of Radiation and Matter Reduced Syllabus Important Questions With Answer Key 2021

12th Standard

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Physics

Time : 01:00:00 Hrs
Total Marks : 105

Multiple Choice Questions

15 x 1 = 15
1. The wavelength λe of an electron and λp of a photon of same energy E are related by

(a)

λp $\alpha$ λe

(b)

${ \lambda }_{ p }\alpha \sqrt { { \lambda }_{ e } }$

(c)

${ \lambda }_{ p }\alpha \frac { 1 }{ \sqrt { { \lambda }_{ e } } }$

(d)

${ \lambda }_{ p }\alpha { \lambda }_{ e }^{ 2 }$

2. If a light of wavelength 330 nm is incident on a metal with work function 3.55 eV, the electrons are emitted. Then the wavelength of the emitted electron is (Take h = 6.6 × 10–34 Js)

(a)

<2.75×10−9m

(b)

≥2.75×10−9m

(c)

≤2.75×10−12m

(d)

<2.5×10−10m

3. A photoelectric surface is illuminated successively by monochromatic light of wavelength λ and $\frac{\lambda}{2}.$ If the maximum kinetic energy of the emitted photoelectrons in the second case is 3 times that in the fi rst case, the work function at the surface of material is

(a)

$\frac{hc}{\lambda}$

(b)

$\frac{2hc}{\lambda}$

(c)

$\frac{hc}{3\lambda}$

(d)

$\frac{hc}{2\lambda}$

4. The threshold wavelength for a metal surface whose photoelectric work function is 3.313 eV is

(a)

4125 $\mathring { A }$

(b)

3750$\mathring { A }$

(c)

6000$\mathring { A }$

(d)

2062.5$\mathring { A }$

5. Emission of electrons by the absorption of heat energy is called_______emission.

(a)

photoelectric

(b)

field

(c)

thermionic

(d)

secondary

6. If the frequency of light in a photoelectron experiment is doubled the stopping potential will

(a)

be doubled

(b)

be halved

(c)

become more than double

(d)

become less than double.

7. A photon of energy hv is absorbed by a free electrons of a metal having work function ф < hv

(a)

The electron is sure to come out

(b)

The electron is sure to come out with a kinetic energy ф < hv

(c)

Either the electron does not come out or it comes out with kinetic energy hv - ф

(d)

It may come out with a kinetic energy less than hv - ф

8. As the intensity of incident light increases

(a)

K.E of emitted photo electrons increases

(b)

photoelectric current decreases

(c)

photoelectric current increases

(d)

K.E of emitted photo electrons decreases

9. Stopping potential of emitted photo electrons is given by (where ф =hv0)

(a)

$\frac { hv-{ \phi }_{ 0 } }{ e }$

(b)

hv-ф

(c)

$\frac { hv }{ e }$

(d)

$\frac { hv+{ \phi }_{ 0 } }{ e }$

10. A photocell employs photoelectric effect to convert

(a)

Change in the frequency of light into a change in electric voltage

(b)

Change in the intensity of light into a change in photoelectric current

(c)

Change in the intensity of light into a change in work function of photocathode

(d)

Change in the frequency of light into a change in electric current

11. Light of wavelength 5000Å falls on a sensitive plate with photoelectric work function of 1.9 eV. The K.E of the photo electron emitted will be

(a)

0.58 eV

(b)

2.48 eV

(c)

1.24 eV

(d)

1.16 eV

12. The particle which has zero mass but has energy is

(a)

electron

(b)

photon

(c)

proton

(d)

neutron

13. The phenomena by which metals emit electrons under the influence of radiation is called ________

(a)

interference

(b)

polarization

(c)

ionization

(d)

photoelectric effect

14. In an electron microscope, the electron beam is associated through a large potential difference in a device called _______

(a)

accelerator

(b)

electron gun

(c)

CRO

(d)

vibrator

15. For a given photo sensitive surface the ratio of stopping potential for three different incident frequencies

(a)

1: 2 : 3

(b)

1: 4: 9

(c)

1: 1 : 1

(d)

$\sqrt { 1 } :\sqrt { 2 } :\sqrt { 3 }$

16. 2 Marks

10 x 2 = 20
17. Why do metals have a large number of free electrons?

18. Define work function of a metal. Give its unit.

19. Give the definition of intensity of light and its unit.

20. How will you define threshold frequency?

21. Write the relationship of de Broglie wavelength λ associated with a particle of mass m in terms of its kinetic energy K.

22. A deuteron and an alpha particle are accelerated with the same potential. Which I one of the two has i) greater value of de Broglie wavelength associated with it and
ii) less kinetic energy? Explain.

23. What is photo emissive cell?

24. What is meant by de Broglie waves?

25. If the intensity of radiation in a photocell is increased how does the stopping potential
vary?

26. A proton and a deuteron have the same velocity, what is the ratio of their de Broglie wavelengths?

27. 3 marks

10 x 3 = 30
28. Briefly discuss the observations of Hertz. Hallwachs and Lenard.

29. Give the construction and working of photo emissive cell.

30. Derive an expression for de Broglie wavelength of electrons.

31. Describe briefly Davisson Germer experiment which demonstrated the wave nature of electrons.

32. Light of wavelength 390 nm is directed at a metal electrode. To find the energy of electrons ejected, an opposing potential difference is established between it and another electrode. The current of photoelectrons from one to the other is stopped completely when the potential difference is 1.10 V. Determine i) the work function of the metal and ii) the maximum wavelength of light that can eject electrons from this metal

33. (a) Define the term 'intensity of radiation' in terms of photon picture of light.
(b) Two monochromatic beams, one red and the other blue; have the same intensity. In which case
(i) the number of photons per unit area per second is larger,
(ii) the maximum kinetic energy of the photoelectrons is more? Justify you answer.

34. Write three characterists features in photoelectric effect which cannot be explained on the basis of wave theory of light, but can be explained only using Einstein's equation.

35. An electron and a proton have the same kinetic energy. Which one of the two has the larger de Broglie wavelength and why?

36. (i) Draw a graph showing variation of photoelectric current (I) with anode potential (V) for different intensities of incident radiation. Name the characteristic of the incident radiation that is kept constant in this experiment.
(ii) If the potential difference used to accelerate electrons is doubled, by what factor does the de-Broglie wavelength associated with the electrons change?

37. Red light however bright it is, cannot produce the emission of electrons from a clean zinc surface, but even weak ultraviolet radiation can do so; why?

38. 5 marks

8 x 5 = 40
39. Calculate the maximum kinetic energy and maximum velocity of the photoelectrons emitted when the stopping potential is 81 V for the photoelectric emission experiment.

40. When a 6000 A light falls on the cathode of a photo cell and produced photoemission. If a stopping potential of 0.8 V is required to stop emission of electron, then determine the
(i) frequency of the light
(ii) energy of the incident photon
(iii) work function of the cathode material
(iv) threshold frequency and
(v) net energy of the electron after it leaves the surface.

41. A 3310 Å photon liberates an electron from a material with energy 3x10-19 J while another 5000 A photon ejects an electron with energy 0.972x 10-19 J from the same material. Determine' the value of Planck's constant and the threshold wavelength of the material.

42. Calculate the de Broglie wavelength of a proton whose kinetic energy is equal to 81.9 x 10-15J. (Given: mass of proton is 1836 times that of electron).

43. The ground state energy of hydrogen atom is -13.6 eV. If an electron makes a transition from an energy level -1.51 eV to -3.4 eV, calculate the wavelength of the spectral line emitted and name the series of hydrogen spectrum to which it belongs.

44. An ∝ - particle and a proton are accelerated from rest through the same potential difference V. Find the ratio of de Broglie wavelength associated with them.

45. What is the stopping potential of a photocell, in which electrons with a maximum kinetic energy of 6 eV are emitted?

46. An electron and a proton, each have de Broglie wavelength of 1.00 nm.
(a) Find the ratio of their momenta.
(b) Compare the kinetic energy of the proton with that of the electron.