Question

In his experiment on photoelectric effect, Robert A. Millikan found the slope of the cut-off voltage versus frequency of incident light plot to be 4.12 × 10⁻¹⁵ Vs. Calculate the value of Planck’s constant from it.

Answer 1

Slope of cut-off voltage (V_s) Vs frequency is given:

Slope = 4.12 × 10⁻¹⁵ Vs

Einstein photoelectric equation:

eV_s = hf − Φ

or, V_s = `(hf)/e - phi/e`

also, Slope = `h/e`

∴ `h/e` = 4.12 × 10⁻¹⁵ Vs

h = 4.12 × 10⁻¹⁵ × 1.6 × 10⁻¹⁹

= 6.592 × 10⁻³⁴ Js

≈ 6.63 × 10⁻³⁴ Js