CBSE Class 12 Physics Chapter-11 Important Questions – Free PDF Download
Free PDF download of Important Questions with Answers for CBSE Class 12 Physics Chapter 11 – Dual Nature of Radiation and Matter 2 Marks Questions prepared by expert Physics teachers from latest edition of CBSE(NCERT) books only by CoolGyan to score more marks in CBSE board examination.
CBSE Class 12 Physics Chapter-11 – Dual Nature of Radiation and Matter
CBSE Class 12 Physics Important Questions Chapter 11 – Dual Nature of Radiation and Matter
2 Marks Questions
1. Derive an expression for debroglie wavelength of an electron?
Ans. If a beam of electrons traveling through a potential difference of V volt, the electron acquires kinetic energy.
Multiply by m
2. Light of wavelength 
falls on an aluminum surface. In aluminum 4.2 eV are required to remove an electron. What is the kinetic energy of (a) fastest (b) the slowest photoelectron?
Ans. 
(a) 
This is the K.E of the fastest electron
(b) Zero
3. An electromagnetic wave of wavelength 
is incident on a photosensitive surface of negligible work function. If the photoelectrons emitted from this surface have de-Broglie wavelength
. Prove that 
Ans. 
Squaring we get
Or
4. It is difficult to remove a free electron from copper than from sodium? Why?
Ans. since 
Where 
is the threshold wavelength
Since 
Work function for copper is greater and it becomes difficult to remove a free electron from copper.
5. Obtain the expression for the maximum kinetic energy of the electrons emitted from a metal surface in terms of the frequency of the incident radiation and the threshold frequency?
Ans. 
W is the threshold energy or work function depends upon threshold frequency 
Or
6. For a given K.E. which of the following has the smallest de–broglie wavelength: electron, proton, 
?
Ans. Debroglie wavelength
When E is energy 
Comparing masses we get mass of is more; hence wavelength of alpha particle is minimum.
7. Photoelectrons are emitted with a maximum speed of 7
105m/s from a surface when light of frequency 81014Hz is incident on it. Find the threshold frequency for this surface?
Ans. 
8. Is photoelectric emission possible at all frequencies? Give reason for your answer?
Ans.No, photoelectric emission is not possible at all frequencies because it is possible only if radiation energy is greater than work function 
of the emitter.
9. Assume that the frequency of the radiation incident on a metal plate is greater than its threshold frequency. How will the following change, if the incident radiation is doubled? (1) Kinetic energy of electrons
(2) Photoelectric current
Ans.(1) If the frequency of the incident radiation is doubled 
is increased, hence kinetic energy is increased.
(2) If the frequency of the incident radiation is doubled there will be no change in the number of photoectrons i.e. photoelectronic current.
10. Why are de – broglie waves associated with a moving football is not visible?
Ans.The wavelength of a wave associated with a moving football is extremely small, which cannot be detected.
Since
11. By how much would the stopping potential for a given photosensitive surface go up if the frequency of the incident radiations were to be increased from 41015 Hz to 8 1015 Hz? ( 
)
Ans. Stopping potential 
12. Work function of Na is 2.3eV. Does sodium show photoelectric emission for light on the velocity of photoelectrons?
Ans.Since 
Velocity of photoelectrons increases with the decrease in the wavelength of the incident light.
13. An electron and an alpha particle have the same debroglie wavelength associated with them? How are their kinetic energies related to each other?
Ans.
Dividing equation (1) and (2)
14. An 
and a proton are accelerated from rest through same potential difference V. find the ratio of de–broglie wavelength associated with them?
Ans.
Now kinetic energy 
15.The photoelectric cut-off voltage in a certain experiment is 1.5 V. What is the maximum kinetic energy of photoelectrons emitted?
Ans.Photoelectric cut-off voltage, 
= 1.5 V
The maximum kinetic energy of the emitted photoelectrons is given as:
Where,
e= Charge on an electron
C
Therefore, the maximum kinetic energy of the photoelectrons emitted in the given experiment is 
J.
16.The threshold frequency for a certain metal is
Hz. If light of frequency
Hz is incident on the metal, predict the cutoff voltage for the photoelectric emission.
Ans.Threshold frequency of the metal, 
Frequency of light incident on the metal, 
Charge on an electron, e
C
Planck’s constant, h
Js
Cut-off voltage for the photoelectric emission from the metal =
The equation for the cut-off energy is given as:
Therefore, the cut-off voltage for the photoelectric emission is 2.0292 V
17.The work function for a certain metal is 4.2 eV. Will this metal give photoelectric emission for incident radiation of wavelength 330 nm?
Ans.No
Work function of the metal, 
Charge on an electron, e 
C
Planck’s constant, h
Js
Wavelength of the incident radiation, 
= 330 nm
m
Speed of light, c
m/s
The energy of the incident photon is given as:
It can be observed that the energy of the incident radiation is less than the work function of the metal. Hence, no photoelectric emission will take place.
