Lesson eLearning

Metal X has a work function of 2.5 eV. Which of the following wavelengths of incident light can cause electrons to be emitted from its surface?

hard MCQ 4 marks Class 12 · Physics · Dual Nature of Radiation and Matter
  1. 800 nm
  2. 600 nm
  3. 500 nm
  4. 400 nm
Answer
400 nm

The correct answer is 400 nm — Thus only option D, 400 nm, can cause photoemission.

Step-by-step solution

  1. Compute the threshold wavelength: where eV·nm and eV.
  2. Calculate nm.
  3. Any photon with wavelength  nm has energy work function and can emit electrons.
  4. Compare each option: 800 nm > 496 nm (no emission), 600 nm > 496 nm (no), 500 nm ≈ 496 nm (just insufficient), 400 nm < 496 nm (sufficient).
  5. Thus only option D, 400 nm, can cause photoemission.
Common mistake: Comparing energies instead of wavelengths, or using eV·nm incorrectly

Concept tested: Photoelectric threshold wavelength

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