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Wave Optics

Question
CBSEENPH12039302

(i) State Bohr's quantization condition for defining stationary orbits. How does the de Broglie hypothesis explain the stationary orbits?

(ii) Find the relation between three wavelengths λ1, λ2 and λ3  from the energy-level diagram shown below.


Solution

Bohr's Quantisation Rule: 
According to Bohr, an electron can revolve only in certain discrete, non-radiating orbits for which the total angular momentum of the revolving electron is an integral multiple of fraction numerator straight h over denominator 2 straight pi end fraction ; where h is the Planck's constant. 
That is,    mvr space equals space fraction numerator nh over denominator 2 straight pi end fraction
b) 
Using Rydberg's formula for spectra of hydrogen atom, we have
1 over straight lambda subscript 1 space equals space R space open parentheses 1 over n subscript 2 squared space minus space 1 over n subscript 3 squared close parentheses space space space space space space space space space space space space... space left parenthesis 1 right parenthesis thin space

1 over straight lambda subscript 2 space equals space R space open parentheses 1 over n subscript 1 squared space minus space 1 over n subscript 2 squared close parentheses space space space space space space space space space space space space... space left parenthesis 2 right parenthesis thin space

1 over straight lambda subscript 3 equals space R space open parentheses 1 over n subscript 1 squared space minus space 1 over n subscript 3 squared close parentheses space space space space space space space space space space space space space... space left parenthesis 3 right parenthesis thin space

Now comma space adding space left parenthesis 1 right parenthesis thin space and space left parenthesis 2 right parenthesis comma space we space get

1 over straight lambda subscript 1 space plus space 1 over straight lambda subscript 2 space equals space space R space open parentheses 1 over n subscript 1 squared space minus space 1 over n subscript 3 squared close parentheses space equals space 1 over straight lambda subscript 3 space semicolon space
Hence, the relation between 3 wavelengths from the energy-level diagram is obtained.