Question
If you increase the intensity of light shining on a metal but keep the frequency below the threshold, will photoelectrons be emitted? Many students intuitively say “yes — more light, more energy”. Why is this wrong?
Solution — Step by Step
Intensity equals (number of photons per unit area per unit time) × (energy per photon). Increasing intensity at fixed frequency means more photons, but each photon still carries the same energy .
For emission, a single photon must carry at least the work function :
If , no individual photon has enough energy to liberate an electron — no matter how many such photons strike.
An electron absorbs one photon at a time. It cannot “save up” energy from multiple sub-threshold photons. This single-photon-per-electron interaction is the heart of the quantum nature of light.
Below threshold frequency, no photoelectrons are emitted regardless of intensity. The threshold is a frequency cutoff, not an energy-flux cutoff.
Final answer: No — emission is impossible below threshold frequency, no matter how intense the light.
Why This Works
This is the foundational conceptual leap from classical to quantum physics. The classical wave picture predicts that enough intensity should always eject electrons. Experiment shows otherwise — emission depends on frequency, not intensity. Einstein’s photon hypothesis explained this in 1905 and won him the Nobel.
The frequency cutoff is the experimental signature that light comes in discrete energy packets.
Alternative Method
Plot stopping potential versus frequency. Below , the line doesn’t exist — no current, no stopping potential, period. Above , the slope is , independent of intensity.
NEET 2024 had a graph-based PYQ asking which curve represents photoelectric current vs intensity at fixed . Answer: a straight line through origin. Intensity controls current magnitude; frequency controls whether current exists at all.
Common Mistake
Treating “more intensity” as “more energy delivered to a single electron”. An electron interacts with only one photon. Intensity controls the number of emission events per second, never the energy of each event.