Important Formulae of Modern Physics - I NEET Notes | EduRev

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NEET : Important Formulae of Modern Physics - I NEET Notes | EduRev

 Page 1


Modern Physics – I 
Important Formulae 
1.  Bohr's Theory 
(i) Bohr's theory is applicable for hydrogen and hydrogen like atoms/ions. For such type 
of atoms/ions number of electron is one. Although atomic numbers may be different,  
eg,  For 
1
H
1
, atomic number Z = 1 
    For He
+
, atomic number Z =2 and 
   For Li
+2
, atomic number Z = 3 .  
  But for all three number of electron is one.  
  (ii) In nth orbit 
   
2
n 2
0
mv 1 (e)(Ze) nh
. and L mvr
t 4 r 2
? ? ?
? ? ?
 
  After solving these two equations we will get following results 
   (a) 
2
n1
r and r
Zm
?? 
   (b) 
0
Z
v and v m
n
?? 
   (c) 
2
2
Z
E and E m
n
??  
   (d) 
H
1
r 0.529 ? Å
 
 
   (e) 
H6
1
c
v 2.2 10 m / s
137
? ? ?  
   (f) 
H
1
E 13.6eV ??  
   (g) K = |E| and U = 2E  
  (iii) Energy of a photon 
hc
E ?
?
. 
    After substituting values of h and c, we get 
   
o
12375
E(in eV)
(in A)
?
?
 
  (iv) Hydrogen spectrum 
     
Page 2


Modern Physics – I 
Important Formulae 
1.  Bohr's Theory 
(i) Bohr's theory is applicable for hydrogen and hydrogen like atoms/ions. For such type 
of atoms/ions number of electron is one. Although atomic numbers may be different,  
eg,  For 
1
H
1
, atomic number Z = 1 
    For He
+
, atomic number Z =2 and 
   For Li
+2
, atomic number Z = 3 .  
  But for all three number of electron is one.  
  (ii) In nth orbit 
   
2
n 2
0
mv 1 (e)(Ze) nh
. and L mvr
t 4 r 2
? ? ?
? ? ?
 
  After solving these two equations we will get following results 
   (a) 
2
n1
r and r
Zm
?? 
   (b) 
0
Z
v and v m
n
?? 
   (c) 
2
2
Z
E and E m
n
??  
   (d) 
H
1
r 0.529 ? Å
 
 
   (e) 
H6
1
c
v 2.2 10 m / s
137
? ? ?  
   (f) 
H
1
E 13.6eV ??  
   (g) K = |E| and U = 2E  
  (iii) Energy of a photon 
hc
E ?
?
. 
    After substituting values of h and c, we get 
   
o
12375
E(in eV)
(in A)
?
?
 
  (iv) Hydrogen spectrum 
     
  (v) In first orbit of hydrogen atom, 
   E = -13.6eV, K = 13.6eV and U=-272eV  
  Similarly, in second orbit, 
    E = -3.4 eV, K = 3.4 eV and U = -6.8 eV 
  (vi) n = 1 is ground state, n = 2 is first excited state and n = 3 is second excited state. 
(vii) If an electron jumps from n
1
 state to n
2
 (< n
1
) state, then wavelength of photon 
emitted in this process will be given by, 
   
12
nn
hc
EE ??
?
 
  or 
12
o
nn
12375
(in A)
(E E )in eV
??
?
 
  Here, 
1
2
n 2
1
13.6 Z
E eV
n
??
? 
  and  
2
2
n 2
2
13.6 Z
E eV
n
??
?   
  (viii) Rhc = 1 Rydberg = 13.6 eV  
  (ix) R = Rydberg constant = 1.09 × 10
7 
m
-1
 
  (x) As n increases 
(a) Angular momentum, time period, potential energy and total energy will 
increase. 
    (b) Speed, kinetic energy and frequency will decrease. 
  (xi) Total number of emission lines from nth state to ground state are, 
n(n 1)
2
?
. 
2.  Matter Wave or de-Broglie Wave 
(i) Every matter particle having some linear momentum is associated with a wave called 
matter wave or de-Broglie wave. 
  (ii) Wavelength of this wave is given by 
   
h h h h
mv 2Km 2qVm
? ? ? ? ?
?
 
  (iii) For an electron, 
o
150
(in A)
V(in volt)
?? 
3.  Electromagnetic Waves  
  (i) EM-waves have dual nature. Particle as well as a wave. 
(ii) Interference, diffraction or polarization can be explained by wave nature of EM 
waves. Photoelectric effect, Compton effect etc is explained by particle nature of EM 
waves. 
Page 3


Modern Physics – I 
Important Formulae 
1.  Bohr's Theory 
(i) Bohr's theory is applicable for hydrogen and hydrogen like atoms/ions. For such type 
of atoms/ions number of electron is one. Although atomic numbers may be different,  
eg,  For 
1
H
1
, atomic number Z = 1 
    For He
+
, atomic number Z =2 and 
   For Li
+2
, atomic number Z = 3 .  
  But for all three number of electron is one.  
  (ii) In nth orbit 
   
2
n 2
0
mv 1 (e)(Ze) nh
. and L mvr
t 4 r 2
? ? ?
? ? ?
 
  After solving these two equations we will get following results 
   (a) 
2
n1
r and r
Zm
?? 
   (b) 
0
Z
v and v m
n
?? 
   (c) 
2
2
Z
E and E m
n
??  
   (d) 
H
1
r 0.529 ? Å
 
 
   (e) 
H6
1
c
v 2.2 10 m / s
137
? ? ?  
   (f) 
H
1
E 13.6eV ??  
   (g) K = |E| and U = 2E  
  (iii) Energy of a photon 
hc
E ?
?
. 
    After substituting values of h and c, we get 
   
o
12375
E(in eV)
(in A)
?
?
 
  (iv) Hydrogen spectrum 
     
  (v) In first orbit of hydrogen atom, 
   E = -13.6eV, K = 13.6eV and U=-272eV  
  Similarly, in second orbit, 
    E = -3.4 eV, K = 3.4 eV and U = -6.8 eV 
  (vi) n = 1 is ground state, n = 2 is first excited state and n = 3 is second excited state. 
(vii) If an electron jumps from n
1
 state to n
2
 (< n
1
) state, then wavelength of photon 
emitted in this process will be given by, 
   
12
nn
hc
EE ??
?
 
  or 
12
o
nn
12375
(in A)
(E E )in eV
??
?
 
  Here, 
1
2
n 2
1
13.6 Z
E eV
n
??
? 
  and  
2
2
n 2
2
13.6 Z
E eV
n
??
?   
  (viii) Rhc = 1 Rydberg = 13.6 eV  
  (ix) R = Rydberg constant = 1.09 × 10
7 
m
-1
 
  (x) As n increases 
(a) Angular momentum, time period, potential energy and total energy will 
increase. 
    (b) Speed, kinetic energy and frequency will decrease. 
  (xi) Total number of emission lines from nth state to ground state are, 
n(n 1)
2
?
. 
2.  Matter Wave or de-Broglie Wave 
(i) Every matter particle having some linear momentum is associated with a wave called 
matter wave or de-Broglie wave. 
  (ii) Wavelength of this wave is given by 
   
h h h h
mv 2Km 2qVm
? ? ? ? ?
?
 
  (iii) For an electron, 
o
150
(in A)
V(in volt)
?? 
3.  Electromagnetic Waves  
  (i) EM-waves have dual nature. Particle as well as a wave. 
(ii) Interference, diffraction or polarization can be explained by wave nature of EM 
waves. Photoelectric effect, Compton effect etc is explained by particle nature of EM 
waves. 
  (iii) E,B and c
??
?
are mutually perpendicular. 
  (iv) E B c ??
??
?
 
  (v) E and B oscillate in same phase. 
  eg,   B
x
 = B
0
 sin( ?t
 
- ky) then 
   E
Z 
= E
0
 sin ( ?t
 
- ky) 
  (vi) 
12
2 f ,T
f
?
? ? ? ? ?
?
 
  (vii) 
0
0
E
c
B
? 
  (viii) Properties of a photon  
  (a) E = hf =
hc
?
  
   (b)E (in eV) = 
o
12375
(in A) ?
 
   (c) Momentum of photon 
Eh
p
c
??
?
 
   (d) Dynamic mass, 
2
E
m
c
? 
  (ix) ?-rays, X-rays, UV rays, visible light, infrared rays, micro waves, radio waves. 
In moving from left to right, energy and frequency of wave decrease but wavelength 
increases. 
  (i) These are electromagnetic waves of high energy, high frequency and low wavelength. 
  (ii) Wavelength of X-rays lies in the range of 1Å to 100 Å. 
  (iii) Cut off wavelength of X-ray spectrum is given by 
   
o
min
12375
(in A)
V(in volt)
?? 
  (iv) Moseley's law for X-ray spectrum is 
     v (z b) ?? 
  Here, the constant b depends on the series 
    b = 1 for K-series and b = 7.4 f or L-series  
  (v)   
Page 4


Modern Physics – I 
Important Formulae 
1.  Bohr's Theory 
(i) Bohr's theory is applicable for hydrogen and hydrogen like atoms/ions. For such type 
of atoms/ions number of electron is one. Although atomic numbers may be different,  
eg,  For 
1
H
1
, atomic number Z = 1 
    For He
+
, atomic number Z =2 and 
   For Li
+2
, atomic number Z = 3 .  
  But for all three number of electron is one.  
  (ii) In nth orbit 
   
2
n 2
0
mv 1 (e)(Ze) nh
. and L mvr
t 4 r 2
? ? ?
? ? ?
 
  After solving these two equations we will get following results 
   (a) 
2
n1
r and r
Zm
?? 
   (b) 
0
Z
v and v m
n
?? 
   (c) 
2
2
Z
E and E m
n
??  
   (d) 
H
1
r 0.529 ? Å
 
 
   (e) 
H6
1
c
v 2.2 10 m / s
137
? ? ?  
   (f) 
H
1
E 13.6eV ??  
   (g) K = |E| and U = 2E  
  (iii) Energy of a photon 
hc
E ?
?
. 
    After substituting values of h and c, we get 
   
o
12375
E(in eV)
(in A)
?
?
 
  (iv) Hydrogen spectrum 
     
  (v) In first orbit of hydrogen atom, 
   E = -13.6eV, K = 13.6eV and U=-272eV  
  Similarly, in second orbit, 
    E = -3.4 eV, K = 3.4 eV and U = -6.8 eV 
  (vi) n = 1 is ground state, n = 2 is first excited state and n = 3 is second excited state. 
(vii) If an electron jumps from n
1
 state to n
2
 (< n
1
) state, then wavelength of photon 
emitted in this process will be given by, 
   
12
nn
hc
EE ??
?
 
  or 
12
o
nn
12375
(in A)
(E E )in eV
??
?
 
  Here, 
1
2
n 2
1
13.6 Z
E eV
n
??
? 
  and  
2
2
n 2
2
13.6 Z
E eV
n
??
?   
  (viii) Rhc = 1 Rydberg = 13.6 eV  
  (ix) R = Rydberg constant = 1.09 × 10
7 
m
-1
 
  (x) As n increases 
(a) Angular momentum, time period, potential energy and total energy will 
increase. 
    (b) Speed, kinetic energy and frequency will decrease. 
  (xi) Total number of emission lines from nth state to ground state are, 
n(n 1)
2
?
. 
2.  Matter Wave or de-Broglie Wave 
(i) Every matter particle having some linear momentum is associated with a wave called 
matter wave or de-Broglie wave. 
  (ii) Wavelength of this wave is given by 
   
h h h h
mv 2Km 2qVm
? ? ? ? ?
?
 
  (iii) For an electron, 
o
150
(in A)
V(in volt)
?? 
3.  Electromagnetic Waves  
  (i) EM-waves have dual nature. Particle as well as a wave. 
(ii) Interference, diffraction or polarization can be explained by wave nature of EM 
waves. Photoelectric effect, Compton effect etc is explained by particle nature of EM 
waves. 
  (iii) E,B and c
??
?
are mutually perpendicular. 
  (iv) E B c ??
??
?
 
  (v) E and B oscillate in same phase. 
  eg,   B
x
 = B
0
 sin( ?t
 
- ky) then 
   E
Z 
= E
0
 sin ( ?t
 
- ky) 
  (vi) 
12
2 f ,T
f
?
? ? ? ? ?
?
 
  (vii) 
0
0
E
c
B
? 
  (viii) Properties of a photon  
  (a) E = hf =
hc
?
  
   (b)E (in eV) = 
o
12375
(in A) ?
 
   (c) Momentum of photon 
Eh
p
c
??
?
 
   (d) Dynamic mass, 
2
E
m
c
? 
  (ix) ?-rays, X-rays, UV rays, visible light, infrared rays, micro waves, radio waves. 
In moving from left to right, energy and frequency of wave decrease but wavelength 
increases. 
  (i) These are electromagnetic waves of high energy, high frequency and low wavelength. 
  (ii) Wavelength of X-rays lies in the range of 1Å to 100 Å. 
  (iii) Cut off wavelength of X-ray spectrum is given by 
   
o
min
12375
(in A)
V(in volt)
?? 
  (iv) Moseley's law for X-ray spectrum is 
     v (z b) ?? 
  Here, the constant b depends on the series 
    b = 1 for K-series and b = 7.4 f or L-series  
  (v)   
  (vi)  
  (vii) 
22
12
1 1 1
(Z b)R
nn
??
? ? ?
??
?
??
    
    Here, n
1 
< n
2
 
  (viii) Continuous X-Ray spectrum 
      
  Here, 
o
min
12375
(in A)
V(in volt)
?? = cut-off wavelength. 
  (ix) 
  This is actual spectrum of X-rays including continuous and characteristic X-rays.  
5.  Photoelectric Effect 
  (i) K
max
= 
2
max
1
mv
2
=E - W = hf - hf
0 
= eV
0
 = 
0
hc hc
?
??
  
  (ii) Graphs K
max
 = hf - W  
     
  Slope
1
 = Slope
2
 = Planck's constant f
01
 = Threshold frequency of metal-1 
  f
02
 = Threshold frequency of metal-2 
  W
1
 = Work function of metal-1 and 
Page 5


Modern Physics – I 
Important Formulae 
1.  Bohr's Theory 
(i) Bohr's theory is applicable for hydrogen and hydrogen like atoms/ions. For such type 
of atoms/ions number of electron is one. Although atomic numbers may be different,  
eg,  For 
1
H
1
, atomic number Z = 1 
    For He
+
, atomic number Z =2 and 
   For Li
+2
, atomic number Z = 3 .  
  But for all three number of electron is one.  
  (ii) In nth orbit 
   
2
n 2
0
mv 1 (e)(Ze) nh
. and L mvr
t 4 r 2
? ? ?
? ? ?
 
  After solving these two equations we will get following results 
   (a) 
2
n1
r and r
Zm
?? 
   (b) 
0
Z
v and v m
n
?? 
   (c) 
2
2
Z
E and E m
n
??  
   (d) 
H
1
r 0.529 ? Å
 
 
   (e) 
H6
1
c
v 2.2 10 m / s
137
? ? ?  
   (f) 
H
1
E 13.6eV ??  
   (g) K = |E| and U = 2E  
  (iii) Energy of a photon 
hc
E ?
?
. 
    After substituting values of h and c, we get 
   
o
12375
E(in eV)
(in A)
?
?
 
  (iv) Hydrogen spectrum 
     
  (v) In first orbit of hydrogen atom, 
   E = -13.6eV, K = 13.6eV and U=-272eV  
  Similarly, in second orbit, 
    E = -3.4 eV, K = 3.4 eV and U = -6.8 eV 
  (vi) n = 1 is ground state, n = 2 is first excited state and n = 3 is second excited state. 
(vii) If an electron jumps from n
1
 state to n
2
 (< n
1
) state, then wavelength of photon 
emitted in this process will be given by, 
   
12
nn
hc
EE ??
?
 
  or 
12
o
nn
12375
(in A)
(E E )in eV
??
?
 
  Here, 
1
2
n 2
1
13.6 Z
E eV
n
??
? 
  and  
2
2
n 2
2
13.6 Z
E eV
n
??
?   
  (viii) Rhc = 1 Rydberg = 13.6 eV  
  (ix) R = Rydberg constant = 1.09 × 10
7 
m
-1
 
  (x) As n increases 
(a) Angular momentum, time period, potential energy and total energy will 
increase. 
    (b) Speed, kinetic energy and frequency will decrease. 
  (xi) Total number of emission lines from nth state to ground state are, 
n(n 1)
2
?
. 
2.  Matter Wave or de-Broglie Wave 
(i) Every matter particle having some linear momentum is associated with a wave called 
matter wave or de-Broglie wave. 
  (ii) Wavelength of this wave is given by 
   
h h h h
mv 2Km 2qVm
? ? ? ? ?
?
 
  (iii) For an electron, 
o
150
(in A)
V(in volt)
?? 
3.  Electromagnetic Waves  
  (i) EM-waves have dual nature. Particle as well as a wave. 
(ii) Interference, diffraction or polarization can be explained by wave nature of EM 
waves. Photoelectric effect, Compton effect etc is explained by particle nature of EM 
waves. 
  (iii) E,B and c
??
?
are mutually perpendicular. 
  (iv) E B c ??
??
?
 
  (v) E and B oscillate in same phase. 
  eg,   B
x
 = B
0
 sin( ?t
 
- ky) then 
   E
Z 
= E
0
 sin ( ?t
 
- ky) 
  (vi) 
12
2 f ,T
f
?
? ? ? ? ?
?
 
  (vii) 
0
0
E
c
B
? 
  (viii) Properties of a photon  
  (a) E = hf =
hc
?
  
   (b)E (in eV) = 
o
12375
(in A) ?
 
   (c) Momentum of photon 
Eh
p
c
??
?
 
   (d) Dynamic mass, 
2
E
m
c
? 
  (ix) ?-rays, X-rays, UV rays, visible light, infrared rays, micro waves, radio waves. 
In moving from left to right, energy and frequency of wave decrease but wavelength 
increases. 
  (i) These are electromagnetic waves of high energy, high frequency and low wavelength. 
  (ii) Wavelength of X-rays lies in the range of 1Å to 100 Å. 
  (iii) Cut off wavelength of X-ray spectrum is given by 
   
o
min
12375
(in A)
V(in volt)
?? 
  (iv) Moseley's law for X-ray spectrum is 
     v (z b) ?? 
  Here, the constant b depends on the series 
    b = 1 for K-series and b = 7.4 f or L-series  
  (v)   
  (vi)  
  (vii) 
22
12
1 1 1
(Z b)R
nn
??
? ? ?
??
?
??
    
    Here, n
1 
< n
2
 
  (viii) Continuous X-Ray spectrum 
      
  Here, 
o
min
12375
(in A)
V(in volt)
?? = cut-off wavelength. 
  (ix) 
  This is actual spectrum of X-rays including continuous and characteristic X-rays.  
5.  Photoelectric Effect 
  (i) K
max
= 
2
max
1
mv
2
=E - W = hf - hf
0 
= eV
0
 = 
0
hc hc
?
??
  
  (ii) Graphs K
max
 = hf - W  
     
  Slope
1
 = Slope
2
 = Planck's constant f
01
 = Threshold frequency of metal-1 
  f
02
 = Threshold frequency of metal-2 
  W
1
 = Work function of metal-1 and 
  W
2
 = Work function of metal-2 
From the equation K
max
 =hf -W, we can see that K
max
 versus f graph is a straight line with 
positive slope h (a universal constant) and negative intercept W (varies metal to metal). 
    
  Slope
1
 = Slope
2
 = 
h
e
= a universal constant 
From the equation, 
0
hW
Vf
ee
??
??
??
??
,
 
we can see that stopping potential V
0
 versus 
frequency f graph is a straight line with positive slope 
h
e
and negative slope 
W
e
.  
  (iii) Threshold wavelength 
0
hc
W
??  
  (iv) Threshold frequency 
0
W
f
h
?  
  (v) Photoelectric current versus potential graph 
 
Saturation current I
2
 depends upon number of photons incident on metallic plate per sec 
and stopping potential depends on the frequency of incident photons. 
  (vi) For photoemission to take place, 
   E ? W, ? = ?
0
 
and 
  (vii) Kinetic energy of photoelectrons varies between 0 and K
max
. 
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