In a photoelectric effect experiment, light of wavelength 250 nm shines on a metal surface. The work function of the metal is 2.5 eV. What is the maximum kinetic energy of the emitted photoelectrons? (Planck constant h = 4.14 × 10⁻¹⁵ eV·s, speed of light c = 3 × 10⁸ m/s)
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Correct answer
C. 2.47 eV
Principle or equation
Einstein's photoelectric equation: K_max = hf - φ, where f = c/λ.
Why this answer is correct
Photon energy E = hc/λ = (4.14 × 10⁻¹⁵ eV·s × 3 × 10⁸ m/s) / (250 × 10⁻⁹ m) = 4.97 eV. Then K_max = 4.97 eV - 2.5 eV = 2.47 eV.
Example
If λ = 400 nm and φ = 2.0 eV, then E = 3.10 eV, so K_max = 1.10 eV.
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