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In a photoelectric experiment, it was fo...

In a photoelectric experiment, it was found that the stopping potential decreases from 1.85 V to 0.82 V as the wavelength of the incident light is varied from 300nm to 400nm. Calculate the value of the Planck constant from these data.

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The maximum kinetic energy of a photoelectron is
` (K_max = hc/lambda - varphi)`
and the stopping potential is
` V= (K_max/e)= (hc/lambda e )- (varphi/ e).`
` If V_1, V_2, are the stopping potentials at wavelengths
lambda_1 and lambda_2 respectively,
` V_1 = (hc/lambda_1 e)- (varphi/e)`
` and V_2= (hc/ (lambda_2 e)-(varphi/ e)).`
` This gives, V_1- V_2 = (hc/e) ( 1/lambda_1) - (1 / lambda_2)
` or, h= (e((V_1)-(V_2))/ c(1/lambda_1)-(1/lambda_2))`
` = (e(1.85 V - 0.82V)/c(1/(300 xx (10^-9)m))-(1/400 xx (10^-9)m))`
` = (1.03e V/ ((3 xx (10^8)m (s^-1))(1/12 xx (10^7)(m^-1)))`
` = 4.12 xx (10^-15) eVs.`
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