Modern Physics I (Part - 2) - Physics, Solution by DC Pandey NEET Notes | EduRev

DC Pandey (Questions & Solutions) of Physics: NEET

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NEET : Modern Physics I (Part - 2) - Physics, Solution by DC Pandey NEET Notes | EduRev

 Page 1


Introductory Exercise 30.2 
Q 1.  A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength 
2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of 
silver is 4.3 eV. 
Q 2.  1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce 
photo electrons, find the current in the cell. 
Q 3.  Is it correct to say that K
max
 is proportional to f ? If not, what would a correct statement of the 
relationship between K
max
 and f ? 
Q 4.  Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the 
minimum and maximum kinetic energy of the photoelectrons. 
Q 5.  When a metal is illuminated with light of frequency f the maximum kinetic energy of the 
photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy 
increases to 4.2 eV. What is the threshold frequency for this metal. 
 
Solutions 
1.   K
max
(in eV) = E - W 
    
   = 1.9 eV  
  Therefore stopping potential is 1.9 V. 
2.  Number of photons incident per second. 
     
  Number of electrons emitted per second = 0.1% of 
    
  ?  Current = Charge (on photoelectrons per second) 
    
    
   = 0.48 × 10
-6
 A = 0.48 ?A
 
 
3.  K
max 
= E - W = hf - hf
0
 = h(f - f
0
) 
   K
max
 ? (f - f
0
) 
4.  K
min
=0 and K
max 
= E - W 
Page 2


Introductory Exercise 30.2 
Q 1.  A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength 
2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of 
silver is 4.3 eV. 
Q 2.  1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce 
photo electrons, find the current in the cell. 
Q 3.  Is it correct to say that K
max
 is proportional to f ? If not, what would a correct statement of the 
relationship between K
max
 and f ? 
Q 4.  Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the 
minimum and maximum kinetic energy of the photoelectrons. 
Q 5.  When a metal is illuminated with light of frequency f the maximum kinetic energy of the 
photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy 
increases to 4.2 eV. What is the threshold frequency for this metal. 
 
Solutions 
1.   K
max
(in eV) = E - W 
    
   = 1.9 eV  
  Therefore stopping potential is 1.9 V. 
2.  Number of photons incident per second. 
     
  Number of electrons emitted per second = 0.1% of 
    
  ?  Current = Charge (on photoelectrons per second) 
    
    
   = 0.48 × 10
-6
 A = 0.48 ?A
 
 
3.  K
max 
= E - W = hf - hf
0
 = h(f - f
0
) 
   K
max
 ? (f - f
0
) 
4.  K
min
=0 and K
max 
= E - W 
    -3.0 ? 3.19 eV 
5.  K
max
 =E - W 
   1.2 = E - W   ...(i) 
   4.2 = 1.5 E - W  ...(ii) 
  Solving this equation, we get W = 4.8 eV = hf
0
 
   
   = 1.16 × 10
15 
Hz 
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