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When light of wavelength λ falls on a metal surface whose work function is φ, the greatest speed attained by the emitted electron is given by

1. √(2(hc + λφ)/(mλ))
2. √(2(hc + λφ)/m)
3. √(2(hc − λφ)/(mλ))
4. √(2(hλ − φ)/m)

Correct answer: √(2(hc − λφ)/(mλ))

Solution

Einstein's equation: (1/2)mv^2 = hc/lambda - phi. Solving, v = sqrt(2(hc/lambda - phi)/m) = sqrt(2(hc - lambda*phi)/(m*lambda)).

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