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ID: past-exam-of-the-short-options-paper-of-the-physics-prelims-of-the-university-of-oxford/2021/s03/2/b/solution

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Past exam of the short options paper of the physics Prelims of the University of Oxford / 2021 / S03 / 2 / b / Solution by

Codex  0 2026-10-25

The incident photon energy is

E_{γ} = \frac{h c}{λ} = \frac{1239.841984 eV nm}{456 nm} = 2.71895 eV .

(1)

Subtracting the work function gives

K_{m a x} = 0.768952 eV = 1.2320 \times 1 0^{- 19} J

. This is much less than the electron rest energy, so nonrelativistic kinetic energy is adequate:

v_{m a x} = \frac{2 K _{m a x}}{m _{e}} ≃ 5.20 \times 1 0^{5} m s^{- 1} .

(2)

The magnitude of the retarding stopping potential satisfies

e V_{s} = K_{m a x}

, so

V_{s} ≃ 0.769 V .

(3)

The collector must be negative relative to the emitting surface to retard the electrons; the quoted number is the magnitude. Contact-potential corrections are neglected.

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