A physicist is studying the photoelectric effect using a clean caesium metal surface. They use a source that emits red light of very high intensity and find that no electrons are emitted from the caesium surface. The physicist then replaces the red light with a source that emits blue light of much lower intensity and finds that electrons are emitted immediately. Explain why electrons are emitted with the blue light but not with the red light, even though the red light has a much higher intensity.

Light reaches the surface as photons, each carrying a fixed amount of energy E=hf, where f is the light frequency.

The intensity of a beam is proportional to the number of photons per unit time, not to the energy of each photon.

Blue light has a higher frequency than red light, so each blue photon has a larger energy.

An electron can be emitted only if a single photon supplies energy greater than the work function (ϕ) of caesium.

Red photons have insufficient energy (hf_red<ϕ) and therefore cannot eject electrons, no matter how many photons are present; increasing intensity does not help because photons cannot combine their energies.

Thus electrons are emitted with blue light but not with red light.