A photocell has a caesium cathode with work function eV

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Photodetection, photonic sources and quantum optics · Photodetection, photonic sources and quantum optics

Optics and Photonics, Ch The photon

Problem

A photocell has a caesium cathode with work function

W = 2.1

eV. It is illuminated by violet light of wavelength

400

nm, producing a photocurrent whose stopping potential is measured to be

V_{s1}

. The lamp's irradiance at the cathode is then increased fivefold, with the wavelength unchanged, and the stopping potential is measured again as

V_{s2}

. A student argues: "brighter light carries more energy per second, so the photoelectrons must leave with more kinetic energy, and

V_{s2} > V_{s1}

." ((a)) Compute the photon energy at

400

nm in eV, and hence

K_{\max}

and

V_{s1}

. ((b)) Compute

V_{s2}

after the fivefold increase in irradiance, and say whether the student's reasoning is correct. ((c)) State, in one sentence, what does change when the irradiance is increased fivefold.

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