Photoelectric Effect and Wave Theory of Light
1. The Wave Theory of Light
Before the 20th century, light was believed to be a wave, based on works of scientists like Huygens
and Maxwell. According to classical wave theory, light is an electromagnetic wave that carries
energy, and its intensity determines how much energy it delivers. Wave theory predicted:
- Higher intensity should eject more electrons.
- Electron emission should have a delay as energy accumulates.
- Frequency should not matter if intensity is high enough.
However, experiments showed these predictions were incorrect.
2. Experimental Observations of the Photoelectric Effect
Experiments by Hertz, Lenard, and Millikan showed:
- Electrons are emitted only if light frequency is above a threshold.
- Emission is instantaneous, with no delay.
- Higher frequency increases electron energy, but intensity only increases the number of electrons.
These findings contradicted wave theory.
3. Einstein's Quantum Explanation (1905)
Einstein proposed that light consists of photons, each carrying energy E = h*f (Planck's constant *
frequency). If a photon's energy exceeds the metal's work function (phi), an electron is ejected
instantly with kinetic energy:
K_max = h*f - phi
This explained why frequency, not intensity, determines electron emission. Einstein won the 1921
Nobel Prize for this work.
4. Summary of Wave Theory vs. Quantum Theory
Wave Theory:
- Light is a continuous wave.
- Intensity affects energy absorption.
- Emission should be delayed and depend on intensity.
Quantum Theory (Einstein's Explanation):
- Light is made of discrete photons.
- Frequency determines energy, not intensity.
- Electron emission is instantaneous if energy exceeds threshold.
1. The Wave Theory of Light
Before the 20th century, light was believed to be a wave, based on works of scientists like Huygens
and Maxwell. According to classical wave theory, light is an electromagnetic wave that carries
energy, and its intensity determines how much energy it delivers. Wave theory predicted:
- Higher intensity should eject more electrons.
- Electron emission should have a delay as energy accumulates.
- Frequency should not matter if intensity is high enough.
However, experiments showed these predictions were incorrect.
2. Experimental Observations of the Photoelectric Effect
Experiments by Hertz, Lenard, and Millikan showed:
- Electrons are emitted only if light frequency is above a threshold.
- Emission is instantaneous, with no delay.
- Higher frequency increases electron energy, but intensity only increases the number of electrons.
These findings contradicted wave theory.
3. Einstein's Quantum Explanation (1905)
Einstein proposed that light consists of photons, each carrying energy E = h*f (Planck's constant *
frequency). If a photon's energy exceeds the metal's work function (phi), an electron is ejected
instantly with kinetic energy:
K_max = h*f - phi
This explained why frequency, not intensity, determines electron emission. Einstein won the 1921
Nobel Prize for this work.
4. Summary of Wave Theory vs. Quantum Theory
Wave Theory:
- Light is a continuous wave.
- Intensity affects energy absorption.
- Emission should be delayed and depend on intensity.
Quantum Theory (Einstein's Explanation):
- Light is made of discrete photons.
- Frequency determines energy, not intensity.
- Electron emission is instantaneous if energy exceeds threshold.
