XII Physics Question Bank-pages-37

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Previous Year CBSE Questions Page 361 of 484 
 
 Answer Key 
Section-B 
 
1 Mark’s Questions 
Q22. (b) Both statement I and II are true; but even it radiation of single wavelength is incident on 
photosensitive surface, electrons of different KE will be emitted. 
Q.23(b) 
Q.24 (d) Photoelectric saturation current is independent of frequency. It only depends on intensity 
of light 
Q.25. inversely 
Q.26. Heinrich Hertz, 1887 
Q.27 (d) All of these 
Q.28 (a) 5.47 Å 
Q.29. (b) λ/2 
Q.30. (b) 
Q.31. (a) frequency 
CASE STUDY BASED QUESTIONS 
Q.32. I. d II. C III. A IV. B V. d 
 
 Two Mark’s Questions 
 
Q.33. Calculating (i) Energy of a photon = hν= 6.63 × 10–34 × 6.0 × 1014 J = 3.978 × 10–19 J 
(ii) Number of photons emitted per second = Power x Energy of photon 
= 2 𝑥10−3 x 3.978 𝑥 10−19 
= 5.03 × 1015 photons/second 
Q.34. If radiation of frequency (ν) greater than threshold frequency (ν0) irradiate the metal surface, 
electrons are emitted out from the metal. So Einstein’s photoelectric equation can be given as 
𝐾max= ½ m𝑣 2 = hν – h𝜈𝑂 
 Characteristic properties of photons: 
(Q) Energy of photon is directly proportional to the frequency (or inversely proportional to the 
wavelength. 
Previous Year CBSE Questions Page 362 of 484 
 
 (ii) In photon-electron collision, total energy and momentum of the system of two constituents 
remains constant. 
Q.35. 
 
 
3 Mark’s Questions 
 
Q.36 (a) The minimum amount of energy required to take out an electron from the surface of metal. 
It is measured in electron volt (eV). 
(b) (i) Saturation current depends only on the intensity of incident 
radiation but is independent of the frequency of incident radiation. 
 (ii) 
Stopping potential does not depend on the intensity of incident 
 radiations. 
 
(iii) Photoelectric current is directly proportional to the 
intensity of incident radiations, provided the given frequency 
is greater than the threshold frequency. 
 
Previous Year CBSE Questions Page 363 of 484 
 
Q.37 
 Slope of the graph 
 
Einstein photoelectric equation 
 
 
Differentiating equation (1) 
 
 
Thus slope is equal to the ratio of Planck’s constant to the charge on electron. 
Threshold frequency – The minimum values of frequency of the incident light below which 
photoelectric emission is not possible is called as threshold frequency. 
Q.38 (a) Wave nature of radiation cannot explain the following: 
(i) The instantaneous ejection of photoelectrons. 
(ii) The existence of threshold frequency for a metal surface. 
(iii) The fact that kinetic energy of the emitted electrons is independent of the intensity of 
light and dependence upon its frequency. 
Thus, the photoelectric effect cannot be explained on the basis of wave nature of light. 
(b) Photon picture of electromagnetic equation is based on particle nature of light. Its basic features 
are: 
(i) In interaction with matter, radiation behaves as if it is made up of particles called photons. 
(ii) Each photon has energy E = h ν and momentum p = h v/c and c, the speed of light 
Previous Year CBSE Questions Page 364 of 484 
 
Q.39. 
(a) ‘X’ is a collector plate potential. (b) 
‘A’ represents the stopping potential. (c) 
Graph for different frequencies 
 
 
 
 
 
 
 
********** 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Previous Year CBSE Questions Page 365 of 484 
 
SECTION-C 
1 Mark Questions 
Directions: These questions consist of two statements, each printed as Assertion and Reason. 
While answering these questions, you are required to choose any one of the following four 
responses. 
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the 
Assertion. 
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the 
Assertion. 
(c)If the Assertion is correct but Reason is incorrect. 
(d) If both the Assertion and Reason are incorrect. 
I If the Assertion is false but Reason is correct 
 
Q.40 Assertion: When the speed of an electron increases its specific charge decreases. 
Reason: Specific charge is the ratio of the charge to mass. 
Q.41 Assertion: Photosensitivity of a metal is high if its work function is small. 
Reason: Work function =hf0, where f0 is the threshold frequency. [AIIMS 1997] 
Q.42 Assertion: Though light of a single frequency (monochromatic) is incident on a metal, the 
energies of emitted photoelectrons are different. 
Reason: The energy of electrons emitted from inside the metal surface, is lost in collision with the 
other atoms in the metal. 
Q.43. Assertion (A): The photoelectrons produced by a monochromatic light beam incident on a 
metal surface have a spread in their kinetic energies. 
ReasonI: The energy of electrons emitted from inside the metal surface, is lost in collision with 
the other atoms in the metal. (CBSE Sample paper 2022) 
Q.44. A proton and an alpha particle are accelerated through the same potential difference. The 
ratio of their de-Broglie wavelengths will be………. 
Q.45. The Davison and Germer experiment established the existence of……………. 
Q.46 The photoelectric current is unaffected by 
1. incident light frequency 
Previous Year CBSE Questions Page 366 of 484 
 
2.metal work function 
3.stopping potential 
4.incident light intensity 
(a) (i) and (iv) only 
(b) (ii) and (iii) only 
I (iii) only 
(d) (ii) only 
Q.47. Which of the following will emit photoelectrons when it collides with a metal? 
(a) UV radiations (b) Infrared radiation (c) Radio waves (d) Microwaves 
Q.48. What will be the de-Broglie wavelength of an electron accelerated from rest through a 
potential difference of 100 volts? 
(a) 12.3 Å (b) 1.23 Å (c) 0.123 Å (d) None of these 
Q.49. Two beams, one of red light and the other of blue light having the same intensity are incident 
on a metallic surface to emit photoelectrons. Which emits electrons of greater frequency? 
(a) Both (b) Red light (c) Blue light (d) None 
Q.50. In the Davisson and Germer experiment, the velocity of electrons emitted from the electron 
gun can be increased by 
(a)Increasing the potential difference between the anode and filament 
(b) Increasing the flame current 
(c) Decreasing the flame current 
(d) Decreasing the potential difference between anode and filament 
Q.51. The work function for a metal surface is 4.14 eV. The threshold wavelength for this metal 
surface is: 
 (a) 4125 Å (b) 2062.5 Å (c) 3000 Å (d) 6000 Å 
 ( CBSE Sample paper 2022) 
 
CASE STUDY BASED QUESTIONS 
Q.52 Lenard observed that when ultraviolet radiations were allowed to fall on the emitter plate of 
an evacuated glass tube, enclosing two electrodes (metal plates), current started flowing in the 
circuit connecting the plates. As soon as the ultraviolet radiations were stopped, the current flow 
https://collegedunia.com/exams/davisson-and-germer-experiment-set-up-observations-and-de-broglies-relation-physics-articleid-103
Previous Year CBSE Questions Page 367 of 484 
 
also stopped. These observations proved that it was ultraviolet radiations, falling on the emitter 
plate, that ejected some charged particles from the emitter and the positive plate attracted them. 
(I)Alkali metals like Li, Na, K and Cs show photo electric effect with visible light but metals like 
Zn, Cd and Mg respond to ultraviolet light. Why? 
(a) Frequency of visible light is more than that for ultraviolet light(b) Frequency of visible light is less than that for ultraviolet light 
I Frequency of visible light is same for ultraviolet light 
(d) Stopping potential for visible light is more than that for ultraviolet light 
(II)Why do we not observe the phenomenon of photoelectric effect with non-metals? 
(a)For non-metals the work function is high 
(b) Work function is low 
I Work function can’t be calculated 
(d) For non-metals, threshold frequency is low 
(III)What is the effect of increase in intensity on photoelectric current? 
(a)Photoelectric current increases 
(b) Decreases 
(c) No change 
(d) Varies with the square of intensity 
(IV)Name one factor on which the stopping potential depends 
(a)Work function 
(b) Frequency 
© Current 
(d) Energy of photon 
(V)How does the maximum K.E of the electrons emitted vary with the work function of metal? 
(a) It doesn’t depend on work function 
(b) It decreases as the work function increases 
(c) It increases as the work function increases 
(d) It’s value is doubled with the work function 
Q.53.Name the phenomenon which is used to establish the wave nature of electrons in the 
Davisson-Germer experiment. CBSE (2020) 
Q.54. Calculate the kinetic energy of an electron having de Broglie wavelength of 1Å. 
Previous Year CBSE Questions Page 368 of 484 
 
 CBSE (2017) 
Q.55. Name the three important features in photoelectric effect which can be explained by 
Einstein’s photoelectric equation . CBSE (2017) 
 
TWO MARK’s QUESTIONS 
Q.56. The de-broglie wavelengths associated with an electron and proton are equal. Prove that the 
kinetic energy of the electron is greater than that of then proton. 
Q.57 Two lines, A and B, in the plot given below show the variation of de-Broglie wavelength, λ 
versus 1/√V, Where V is the accelerating potential difference, for two 
particles carrying same charge. Which one of two represents a particle of 
smaller mass? (All India 2008) 
 THREE MARK’s QUESTIONS 
Q.58. A proton and an alpha particle have the same de – Broglie wavelength. Determine the ratio 
of (i) their accelerating potentials (ii) their speeds. CBSE 2015 
Q.59. The graph in figure, shows the variation of stopping potential with frequency v of the 
incident radiation for two photosensitive metals X and Y. 
 
 V0 X Y 
 
 
 
 
 o 0.5 1.0 
 
(i)which of the metals has large threshold wavelength? Give reason. 
(ii) Explain giving reason, which metal gives out electrons, having large K.E. for the same 
wavelength of the incident radiation 
(iii) If the distance between the light source and metal X is doubled, how will the K.E of electrons 
emitted from it change? Give reason. CBSE 2008 
Q.60. a) State three important properties of photons which describe the particle picture of 
electromagnetic radiation. 
(b) Use Einstein’s photoelectric equation to define the terms. (i) stopping potential and 
(ii) threshold frequency. 
Previous Year CBSE Questions Page 369 of 484 
 
SECTION C- Answer Key 
Q.40(b) 
Q.41(b) Less work function means less energy is required for ejecting out the electrons 
Q.42.(a) When a light of single frequency falls on the electrons of inner layer of metal, then this 
electron comes out of the metal surface after a large number of collisions with atom of it’s upper 
layer. 
Q.43 (a) 
Q.44. 2√2 :1 
Q.45. electron wave 
Q.46 (c) (iii) only 
Q.47. (a) UV radiations 
Q.48. (b) 1.23 Å 
Explanation: The value of V is given 100 volts. 
Equation of de Broglie wavelength of electron λ=h/2meV 
λ=6.6×10−34/2(9.1×10−31)(1.6×10−19)(100) 
λ=6.6×10−34/2(9.1×10−31)(1.6×10−19)(100) 
λ = 6.6 ×10-34 / 5.4 ×10-10m 
λ=1.277×1010m=1.23Å 
Q.49. (c) Blue light 
Explanation: Einstein’s photoelectric equation is given by Ek = hv – ϕϕ 
Where, Ek is the maximum kinetic energy of the ejected electron. 
Energy of the photon of the blue light (hv)blue is higher compared to red light (hv)red. In 
photoelectric emission, Ek ∝ (hv). Therefore, blue light emits electrons of greater kinetic energy 
than that of red light. 
Q.50. (a) increasing the potential difference between the anode and filament 
Explanation: if, e is the charge on electron, V is the potential difference, m is the mass of the 
particle and v is the velocity of the particle then, eV = ½ mv2 
Therefore, in the Davisson and Germer experiment, velocity of electrons emitted from electron 
guns can be increased by increasing the potential difference between anode and filament. 
Q.51. (iii) 3000 Å 
Previous Year CBSE Questions Page 370 of 484 
 
Q.52. (I)b (ii) a (III) a (IV) b (V) b 
Q.53. Electron diffraction. 
Q.54. de-Broglie wavelength 
λ = ℎ/𝑚𝑣=h/p 
Kinetic Energy= p2/2m=
1
2𝑚
(
ℎ
λ
)2 = (6.626x10-34)2/2x9.11x10-31x(1x10-10)2 
 = 2.409x10-17J 
Q.55.( i)For a given metal and frequency of incident radiation, the number of photoelectrons 
ejected per second is directly proportional to the intensity of incident radiation. 
(ii) Maximum kinetic energy of the emitted photo electron is independent of the intensity of 
incident radiations. 
(iii) Emission of photoelectrons is instantaneous 
TWO MARK’s QUESTIONS 
 
Q.56. de-broglie wavelength of a particle of mass m and kinetic energy E is 
 λ = 𝒉 √𝟐𝒎𝑬 
 or E= h2/2m λ2 
 for particle having same λ, Eα 1/m 
As mass of electron as smaller than that of proton, so the electron has a higher kinetic energy and 
it is moving faster. 
 
Q.57.