Photoelectric Effects

Experiment:   Expose a metal surface to a light beam then measure the kinetic energy of the ejected electrons from the surface as functions of the intensity and frequency of the incident light beam.

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What classical theory predict

Classical wave theory of light shows that energy of the incident light beam is proportional to the square of the amplitude of the oscillating electric field and thus proportional to the intensity.  Therefore, kinetic energy of the ejected electrons would be proportional to the intensity and independent of  the frequency of the light beam.

What are observed

1. Electrons are ejected only if the frequency of the light exceeds a threshold value characteristic of the metal, and regardless of the incident light intensity.
2. The kinetic energy of the ejected electrons is linearly proportional to the frequency of the incident light but independent of its intensity.

Quantum mechanical interpretations

Based on Plank's hypothesis that light consists of discrete quanta, called photons, of energy , Einstein derived the expression for the kinetic energy of the ejected electrons

where is the work function, i.e. the binding energy of the electron in the metal solid.  is the minimum energy of the incident light required to remove an electron from the metal and thus the threshold frequency for the metal is

Afterthought:   This is another example that light waves behave like particles.  Furthermore, Einstein showed that the Plank's constant is an universal constant, further validated Plank's hypothesis and made it more widely accepted.

-- ThanhTruong - 16 Jul 2007