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Question:

Following results are observed when sodium metal is irradiated with different wavelengths.

Calculate (a) threshold wavelength and, (b) Planck’s constant.

               λ (nm) 500 450

400
       v × 10–5 (cm s–1) 2.55 4.35 5.35

 

Answer:

(a) Let us suppose threshold wavelength to be λD nm (= λ0 x 10-9 m), the kinetic energy of the radiation is given as:

h(v-v0)=1/2 mv2

Or

hc(1/λ - 1/λD) = 1/2 mv2

Threshold wavelength (λD)= 540 nm

 Substituting this value in equation iii,we get

= h  x (3x108)/10-9 [1/400-1/540]

= [(9.11x10-31)(5.20x106)2] / 2

=6.66x10-34Js


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