CBSE Class 12 Physics 2019 Outside Delhi Set 1 Paper

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Question : 8
Total: 27
A photon emitted during the de-excitation of electron from a state n to the first excited state in a hydrogen atom, irradiates a metallic cathode of work function 2eV, in a photo cell, with a stopping potential of 0.55V. Obtain the value of the quantum number of the state n.
OR
A hydrogen atom in the ground state is excited by an electron beam of 12.5eV energy. Find out the maximum number of lines emitted by the atom from its excited state.
Solution:  
● For writing Einstein's photoelectric equation
● For writing, En=
13.6
n2

● For finding the value of n
From photoelectric equation, hv=ϕ0+eVs
=2+0.55=2.55eV
Given, En=
13.6
n2

The energy difference
E=3.4(2.55)eV =0.85eV
13.6
n2
=0.85

n=4
OR
Calculation of energy in excited state Formula
Finding out the maximum number of lines.
Energy in ground state, E1=13.6eV
Energy supplied =12.5eV
Energy in excited state, 13.6+12.5=1.1eV
But, En=
13.6
n2
=1.1

n=3
Maximum number of lines =3
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