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A photoelectric surface is illuminated successively by monochromatic light of wavelength λ and λ/2. If the maximum kinetic energy of the emitted photoelectrons in the second case is 3 times that in the first case, the work function of the surface of the material is (h = Planck's constant, c = speed of light)

1. hc/λ
2. 2hc/λ
3. hc/3λ
4. hc/2λ

Correct answer: hc/3λ

Solution

Using the photoelectric equation, K.E. = hc/λ - ϕ, where ϕ is the work function. For λ and λ/2, the kinetic energies are related as K.E.2 = 3 × K.E.1. Solving these equations gives ϕ = hc/3λ.

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