Modern Physics 1 Question 57

53. Ultraviolet light of wavelengths 800\AA and 700\AA when allowed to fall on hydrogen atoms in their ground state is found to liberate electrons with kinetic energy 1.8eV and 4.0eV respectively. Find the value of Planck’s constant.

(1983,4M)

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Solution:

  1. When 800\AA wavelength falls on hydrogen atom (in ground state) 13.6eV energy is used in liberating the electron. The rest goes to kinetic energy of electron.

Hence, K=E13.6 (in eV ) or

(1.8×1.6×1019)=hc800×101013.6×1.6×1019

Similarly, for the second wavelength :

(4.0×1.6×1019)=hc700×101013.6×1.6×1019

Solving these two equations, we get

h6.6×1034Js



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