In photo-emission, the number of photo-electrons ejected per second depends on the?

1. Analyze the question The question asks what controls the number of photo-electrons emitted per second. It is not asking about their energy or whether emission happens at all.

2. Explain the concept In the photoelectric effect, light behaves as photons, each carrying energy E = hf.

• Whether emission occurs depends on the frequency. It happens only if f ≥ f₀, the threshold frequency, where hf₀ = φ (the work function).
• The maximum kinetic energy of each electron depends on frequency and work function: KE_max = hf − φ.
• The number of electrons per second depends on how many photons hit the metal each second. That is the intensity of the beam. The photocurrent is proportional to the intensity, provided f is above the threshold.

3. Check each option

• A. Frequency of the beam: This affects the energy of each electron, not how many are emitted per second.
• B. Work function: This is a property of the metal. It sets the threshold and the KE_max, not the count.
• C. Threshold frequency: This is also a property of the metal. It decides whether emission occurs at all.
• D. Intensity of the beam: A higher intensity means more photons per second, so more electrons are ejected per second. ✔

4. Answer The number of photo-electrons ejected per second depends on the intensity of the beam, which is option D.

5. Exam tip Remember it as: Intensity = Idea of quantity, Frequency = Force (energy) of each electron.

• Brighter light means more electrons.
• Higher frequency means faster electrons.