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

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

Grade:11

1 Answers

Kevin Nash
askIITians Faculty 332 Points
8 years ago
hc / λ1 – hc / λ0 = K.E. 1 ….(i)
And hc / λ2 – hc / λ0 = K.E.2 ….(ii)
⇒ hc / λ1 - hc / λ2 = K.E.1 – K.E2
⇒ hc [λ2 - λ1 / λ1 λ2] =K.E.1 – K.E2
∴ h = (K.E. – K.E.­21 λ2 / c (λ2 – λ1)
= (1.8 – 4) x 1. 6 x 10-19 x 800 x 10-10 x 700 x 10-10 / 3 x 108 x (700 - 800) x 10-10
= 6.6 x 1034 J.s.

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