Multiple Choice Questions

I Multiple Choice Questions

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

a. λp ∝ λe

b. λp ∝ √λe

c. λp ∝ 1/√λe

d. λp ∝ λ2e

Answer: d)

Solution:

de Broglie wavelength for an electron

λ_e= h / √(2mE) ;

λ_e ∝ 1/√E ;

λ_e² ∝ 1/E  ------------- (1)

For photon

λ_p= hc / E ;

λ_p ∝ 1/E  ------------- (2)

From equation (1) and (2), we have So,

 λ_p ∝ λ_e²

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

Answer: c)

Solution:

deBroglie wavelength λ ∝ 1/√V

 λ₁/λ₂ = √(V₂/V₁) = √(224/14) = √16 = 4

λ₁ = 4λ₂

λ₂ = λ₁/4

3. A particle of mass 3 × 10–6 g has the same wavelength as an electron moving with a velocity 6×106 m s−1 . The velocity of the particle is

a. 1.82×10−18 m s−1

b. 9×10−2 m s−1

c. 3×10−31m s−1

d. 1.82×10−15 m s−1

Answer: d)

Solution:

4. When a metallic surface is illuminated with radiation of wavelength λ, the stopping potential is V. If the same surface is illuminated with radiation of wavelength 2λ, the stopping potential is V/4. The threshold wavelength for the metallic surface is

a. 4λ

b. 5λ

c. 5/2  λ

d. 3λ

Answer: d)

Solution:

5. 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. < 2.75×10−9 m

b. ≥ 2.75×10−9 m

c. ≤ 2.75×10−12 m

d. < 2.5×10−10 m

Answer: b)

Solution:

6. A photoelectric surface is illuminated successively by monochromatic light of wavelength λ and λ/2. If the maximum kinetic energy of the emitted photoelectrons in the second case is 3 times that in the first case, the work function at the surface of material is

a) hc/λ

b) 2hc/ λ

c) hc/3λ

d) hc/2λ

Answer: d)

Solution:

7. In photoelectric emission, a radiation whose frequency is 4 times threshold frequency of a certain metal is incident on the metal. Then the maximum possible velocity of the emitted electron will be

Answer: b)

Solution:

From Enistein's equation

8. Two radiations with photon energies 0.9 eV and 3.3 eV respectively are falling on a metallic surface successively. If the work function of the metal is 0.6 eV, then the ratio of maximum speeds of emitted electrons will be

a) 1:4

b) 1:3

c) 1:1

d) 1:9

Answer: b)

Solution:

9. A light source of wavelength 520 nm emits 1.04 × 1015 photons per second while the second source of 460 nm  produces 1.38 × 1015 photons per second. Then the ratio of power of second source  to that of first source is

a) 1.00

b) 1.02

c) 1.5

d) 0.98

Answer: c)

Solution:

10. The mean wavelength of light from sun is taken to be 550 nm and its mean power is 3.8 × 1026W. The number of photons received by the human eye per second on the average from sunlight is of the order of

a) 1045

b) 1042

c) 1054

d) 1051

Answer: a)

Solution:

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

a) 4125Å

b) 3750 Å

c) 6000 Å

d) 2062.5Å

Answer: b)

Solution:

12. A light of wavelength 500 nm is incident on a sensitive plate of photoelectric work function 1.235 eV. The kinetic energy of the photo electrons emitted is be (Take h = 6.6 × 10–34 Js)

a) 0.58 eV

b) 2.48 eV

c) 1.24 eV

d) 1.16 eV

Answer: c)

Solution:

13. 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

Answer: option d

Answer: d)

Solution:

Centripetal force (F_c.p) = Magnetic force (F_B)

14. 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 Å is/are

a. A only

b. both A and B

c. all these metals

d. none

Answer: b)

Solution:

The energy of the light with the wavelength 4100 × 10^-10 m

Both (A) and (B) emits the electrons, because its work function is less than 3.02eV.

15. Emission of electrons by the absorption of heat energy is called _______ emission.

a. photoelectric

b. field

c. thermionic

d. secondary

Answer: c)

Answers

1. d 2. c 3. d 4. d 5. b 6. d 7. b 8. b 9. c 10. a 11. b 12. c 13. d 14. b 15. C