A student shines a high-intensity green laser onto a negatively charged potassium cathode in a vacuum phototube, but detects no current. However, when they illuminate the cathode with a very dim, low-intensity violet LED, a small photocurrent is registered immediately.
Explain these observations in terms of the photon model of light.
The threshold wavelength (the maximum wavelength of electromagnetic radiation that can cause photoelectric emission) for this potassium cathode is 5.4 × 10-7 m.
Calculate the maximum kinetic energy, in joules, of the emitted photoelectrons when the incident radiation consists of photons each having an energy of 2.9 eV2.9\text{ eV}2.9 eV.
