AS Daily A Level Physics question
Two monochromatic lamps shine normally onto the clean cathode of a photocell. The metal’s threshold wavelength is 500 nm. Each lamp delivers the same power to the same area: Lamp 1 is 400 nm, Lamp 2 is 600 nm. Which statement must be true? (Hint: at equal power, the longer-wavelength source sends more photons per second by the ratio of the wavelengths.)
Answer
The correct answer is C.
Correct: C — Only Lamp 1 produces photoelectrons; Lamp 2 produces none even if it delivers more photons per second at the same power. Since the threshold wavelength is 500 nm, 400 nm light has sufficient photon energy to eject electrons, whereas 600 nm does not, regardless of photon rate (even though at equal power it sends about 600/400 ≈ 1.5 times as many photons per second). A is wrong because being below threshold, 600 nm light cannot eject any electrons no matter how many photons arrive; more photons per second cannot compensate for insufficient photon energy. B is wrong because it assumes 600 nm also causes emission; only 400 nm is above threshold, and while 400 nm would give the larger stopping potential, 600 nm gives zero current. C is correct as stated. D is wrong because maximum kinetic energy (hence stopping potential) depends on photon energy (frequency/wavelength), not on the incident power; 400 nm would produce higher-energy electrons than 600 nm, and 600 nm produces none at all here.