#### 12th Standard Physics English Medium Dual Nature of Radiation and Matter Reduced Syllabus Important Questions 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. In an electron microscope, the electrons are accelerated by a voltage of 14 kV. If the voltage is changed to 224 kV, then the de Broglie wavelength associated with the electrons would

(a)

increase by 2 times

(b)

decrease by 2 times

(c)

decrease by 4 times

(d)

increase by 4 times

3. A particle of mass 3 × 10–6 g has the same wavelength as an electron moving with a velocity 6 10 × 6m s−1. The velocity of the particle is

(a)

1.82x10-18ms-1

(b)

9x10-2ms-1

(c)

3x10-15ms-1

(d)

1.82x10-15ms-1

4. Photons of wavelength λ are incident on a metal. The most energetic electrons ejected from the metal are bent into a circular arc of radius R by a perpendicular magnetic field having magnitude B. The work function of the metal is

(a)

$\frac { hc }{ \lambda } -{ m }_{ e }+\frac { e^{ 2 }{ B }^{ 2 }{ R }^{ 2 } }{ { 2m }_{ e } }$

(b)

$\frac { hc }{ \lambda } +{ 2m }_{ e }{ \left[ \frac { eBR }{ { 2m }_{ e } } \right] }^{ 2 }$

(c)

$\\ \frac { hc }{ \lambda } -{ m }_{ e }{ c }^{ 2 }-\frac { e^{ 2 }{ B }^{ 2 }{ R }^{ 2 } }{ { 2m }_{ e } }$

(d)

$\frac { hc }{ \lambda } -{ 2m }_{ e }{ \left[ \frac { eBR }{ { 2m }_{ e } } \right] }^{ 2 }$

5. The work functions for metals A, B and C are 1.92 eV, 2.0 eV and 5.0 eV respectively. The metals which will emit photoelectrons for a radiation of wavelength 4100$\\ \\ \mathring { A } \\$ is/are

(a)

A only

(b)

both A and B

(c)

all these metals

(d)

none

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. When the intensity of a light source is increased

(a)

the number of photons emitted by the source in unit time increases

(b)

more energetic photons are emitted

(c)

faster photons are emitted

(d)

total energy of the photons emitted per unit time decreases.

8. According to Einstein's photoelectric equation. the plot of the K.E of the emitted photoelectrons from a metal vs the frequency of incident radiation gives a straight line where slope

(a)

depends on the nature ofthe metal used

(b)

depends on the intensity of the radiation

(c)

both a & b

(d)

Depends on neither a or b

9. 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

10. When intensity of incident light increases

(a)

photo current increases

(b)

photo current decreases

(c)

K.E of photoelectrons increases

(d)

K.E of photoelectrons decrease

11. If the radius of third Bohr orbit in hydrogen atom is r, then de-Broglie wavelength of electron in this orbit is

(a)

$\frac { r }{ 3 }$

(b)

3r

(c)

$\frac { 2\pi r }{ 3 }$

(d)

3(2πr)

12. Two photons. each of energy 2.5 eV are simultaneously incident on the metal surface. If the work function of the metal is 4.5 eV then from the surface of the metal

(a)

one electron will be emitted

(b)

two electrons will be emitted

(c)

more than two electrons will be emitted

(d)

not a single electron will be emitted

13. The wave particle duality was extended to particles as "Matter Waves" by _______

(a)

Einstein

(b)

Sommerfeld

(c)

de Broglie

(d)

Louis Pasteur

14. Photo-electric effect was first discovered by _______

(a)

Einstein

(b)

Newton

(c)

Hertz

(d)

Germer

15. The resolving power of a microscope is limited by the ______ of the radiation used.

(a)

wavelength

(b)

intensity

(c)

amplitude

(d)

time of travel

16. 2 Marks
10 x 2 = 20
17. Why do metals have a large number of free electrons?

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

19. Why we do not see the wave properties of a baseball?

20. An electron and an alpha particle havesame kinetic energy. How are the de Broglie wavelengths associated with them related?

21. What is photo voltaic cell?

22. What is meant by de Broglie waves?

23. Two beams, one of red light and the other of blue light, of the same intensity are incident on a metallic surface to emit photoelectrons. Which one of the two beams emits electrons of greater kinetic energy?

24. Ultraviolet light is incident on two photosensitive materials having work functions W1 and W2 (W1 > W2). In which case will the kinetic energy of the emitted electrons be greater? Why?

25. With what purpose was famous Davisson-Germer experiment with electrons performed?

26. The de Broglie wavelength associated with an electron accelerated through a potential difference V is λ. What will be its wavelength when the accelerating potential is increased
to 4V?

27. 3 marks
10 x 3 = 30
28. What do you mean by electron emission? Explain briefly various methods of electron emission.

29. Briefly discuss the observations of Hertz. Hallwachs and Lenard.

30. Explain the effect of potential difference on photoelectric current.

31. List out the laws of photoelectric effect.

32. Explain experimentally observed facts of photoelectric effect with the help of Einstein's explanation.

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

34. For the photoelectric emission from cesium, show that wave theory predicts that
i) maximum kinetic energy of the photoelectrons (Kmax) depends on the intensity I of the incident light.
ii) Kmax does not depend on the frequency of the incident light and
iii) the time interval between the incidence of light and the ejection of photoelectrons is very long.
(Given: The work function for cesium is 2.14 eV and the power absorbed per unit area is 1.60 x 10-6 Wm-2 which produces a measurable photocurrent in cesium.)

35. (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.

36. Show the variation of photocurrent with collector plate potential for different frequencies but same intensity of incident radiation.

37. The figure shows a plot of three curves a, b, c showing the variation of photocurrent vs. Collector plate potential for three different intensities I1,I2, and I3having frequencies v1' v2 and v3 respective incident on a photosensitive surface.
The figure shows a plot of three curves a, b, c showing the variation of photocurrent vs. Collector plate potential for three different intensities I1,I2, and I3 having frequencies v1,v2 and v3 respective incident on a photosensitive surface.

38. 5 marks
8 x 5 = 40
39. How many photons per second emanate from a 50 mW laser of 640 nm?

40. UV light of wavelength 1800 Å is incident on a lithium surface whose threshold wavelength 4965 Å. Determine the maximum energy of the electron emitted.

41. Calculate the momentum and the de Broglie wavelength in the following cases:
i) an electron with kinetic energy 2 eV.
ii) a bullet of 50 g fired from rifle with a speed of 200 m/s
iii) a 4000 kg car moving along the highways at 50 m/s
Hence show that the wave nature of matter is important at the atomic level but is not really relevant at macroscopic level.

42. An electron and a proton, each have the de Brogile wavelength of 1.00 nm.
(i) Find the ratio of their momenta.
(ii) Compare the kinetic energy of the proton with that of the electron.

43. An electron microscope uses electrons accelerated by a voltage of 50 kV. Determine the de Broglie wavelength associated with the electrons. If other factors (such as numerical aperture, etc.) are taken to be roughly the same, how does the resolving power of an electron microscope compare with that of an optical microscope which uses yellow light (A = 5.9 x 10-7 m).

44. If h is Planck's constant. find the momentum of a photon of wavelength 0.1 Ă.

45. Ultraviolet light of wavelength 2271 Ă from a 100 W mercury source irradiates a photocell made of molybdenum metal. If the stopping potential is 1.3 volt, estimate the work functio of the metal. How would the photocell respond to a high intensity ( = 105 W m-2) red light of wavelength 6328 Ă produced by He-Ne laser?

46. For photo electronic effect in sodium, the figure shows the plot of cut-off voltage versus frequency of incident radiation. Calculate
(i) threshold frequency
(ii) work function for sodium.