CBSE Class 12 Physics 2016 Delhi Set 1 Paper

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Question : 21
Total: 26
(i) State Bohr's quantization condition for defining stationary orbits. How does de Broglie hypothesis explain the stationary orbits?
(ii) Find the relation between the three wavelength λ1,λ2 and λ3 from the energy level diagram shown below.
Solution:  
(i) Statement of Bohr's quantization condition
de- Broglie explanation of stationary orbits
(ii) Relation between λ1,λ2λ3
(i) Only those orbits are stable for which the angular momentum, of revolving electron, is an integral multiple of
h
2π
.
[Alternatively
L=
nh
2π
i.e. angular momentum of orbiting electron is quantized.]
According to de-Broglie hypothesis
Linear momentum (p)=
h
λ

And for circular orbit L=rnp where ' r ' is the radius of quantized orbits.
=
rh
λ
Also L=
nh
2π

rh
λ
=
nh
2π

2πrn=nλ
Circumference of permitted orbits are integral multiples of the wave-length λ.
(ii) ECEB=
hc
λ1
.......(i)
EBEA=
hc
λ2
.......(ii)
ECEA=
hc
λ3
.......(iii)
Adding (i) & (ii)
ECEA=
hc
λ1
+
hc
λ2
........(iv)
Using equation (iii) and (iv)
hc
λ3
=
hc
λ1
+
hc
λ2

1
λ3
=
1
λ1
+
1
λ2
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