Photoelectric threshold wavelength
A metal has a work function of 2.48 eV (hc ≈ 1,240 eV·nm). Which wavelength of light will NOT eject electrons, however intense?
Answer D: explanation for every choice
A
250 nm photons carry 4.96 eV, well above the work function.
B
400 nm photons carry 3.1 eV, enough to eject electrons.
C
450 nm photons carry 2.76 eV, enough to eject electrons.
D
Correct. The threshold wavelength is hc/Φ = 1,240/2.48 = 500 nm. Longer wavelengths carry too little energy per photon.
TakeawayThreshold λ = hc/Φ. Longer wavelengths fail at any intensity.
Equations usedPhotoelectric effectPhoton energy
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