1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1% of the incident photons produce photoelectrons, the current in the cell is:
Correct answer: A. 0.48 μA
- A. 0.48 μA
- B. 0.42 mA
- C. 0.48 mA
- D. 0.42 μA
Explanation
Photon energy E = hc/λ = 1240/400 ≈ 3.1 eV = 4.96 × 10⁻¹⁹ J. Number of photons n = P/E = (1.5 × 10⁻³)/(4.96 × 10⁻¹⁹) ≈ 3.02 × 10¹⁵. Photoelectrons = 0.001n. Current = nq = (0.001 × 3.02 × 10¹⁵) × (1.6 × 10⁻¹⁹) ≈ 0.48 μA.
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Classical physics cannot explain blackbody radiation and related observations, leading to Planck’s quantum theory, in which energy is emitted or absorbed in discrete packets. Photons provide the particle model of light, with energy and momentum linked to frequency and wavelength, including the photoelectric effect and its threshold frequency.
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