What Is the Photoelectric Effect and Why Couldn't Classical Wave Theory Explain It?

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A metal requires a minimum energy, called the work function $\Phi$, to release an electron. Conservation of energy gives Einstein's photoelectric equation:

The threshold frequency $f_0$ is the minimum frequency that can cause emission. At this frequency, the emitted electron has zero maximum kinetic energy, so $\Phi=hf_0$.

A common misconception is that brighter light always ejects more energetic electrons. This is false: above the threshold frequency, greater intensity increases the number of emitted electrons per second, while greater frequency increases their maximum kinetic energy.

Exam Technique

For an IB Explain question, link each experimental observation to the failure of classical wave theory and then use $E_{\text{max}}=hf-\Phi$. State explicitly that the photoelectric effect provides evidence for the particle nature of light.