Ultraviolet light with a wavelength of 280 nm is used in an experiment on the photoelectric effect with a lithium cathode (work function, W_o = 2.5…

Question

Ultraviolet light with a wavelength of 280 nm is used in an experiment on the photoelectric effect with a lithium cathode (work function, W_o = 2.5 eV). Find the stopping potential for the photoelectrons.

[h = 6.6 × 10⁻³⁴ Js, c = 3 × 10⁸ ms⁻¹, 1 eV = 1.6 × 10⁻¹⁹ J]

Answer 1

Ultraviolet light with a wavelength of 280 nm is used in an experiment on the photoelectric effect with a lithium cathode (work function, W_o = 2.5 eV). Find the stopping potential for the photoelectrons.

[h = 6.6 × 10⁻³⁴ Js, c = 3 × 10⁸ ms⁻¹, 1 eV = 1.6 × 10⁻¹⁹ J]

4.5 V
3.7 V
1.9 V ✓
2.8 V

Explanation

Photon energy = hc/λ = (6.6 × 10⁻³⁴ × 3 × 10⁸) ÷ (280 × 10⁻⁹) = 7.07 × 10⁻¹⁹ J. Divide by 1.6 × 10⁻¹⁹ to get 4.42 eV.

Maximum kinetic energy = 4.42 − 2.5 = 1.92 eV. The stopping potential in volts has the same number: about 1.9 V.

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