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Question
In an experiment on photoelectric effect, the stopping potential is measured for monochromatic light beams corresponding to different wavelengths. The data collected are as follows:-
Wavelength (nm): 350 400 450 500 550
Stopping potential (V): 1.45 1.00 0.66 0.38 0.16
Plot the stopping potential against inverse of wavelength (1/λ) on a graph paper and find (a) Planck's constant (b) the work function of the emitter and (c) the threshold wavelength.
(Use h = 6.63 × 10-34J-s = 4.14 × 10-15 eV-s, c = 3 × 108 m/s and me = 9.1 × 10-31kg)
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Solution
(a)
When λ = 350, Vs = 1.45
and when λ = 400 , `V_s = 1`
`therefore (hc)/350 = w + 1.45 .......(1)`
and `(hc)/400 = w + 1 .......(2)`
Subtracting (2) from (1) and solving to get the value of h, we get :
`h = 4.2 xx 10^-15 "eV-s"`
(b) Now, work function,
`w = 12240/350 - 1.45 = 2.15 "ev"`
(c) `w = (nc)/λ`
`⇒ λ_"threshold" = (hc)/w`
`⇒ λ_"threshold" = 1240/1.15`
`⇒ λ_"threshold" = 576.8 "nm"`
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