The photoelectric effect is a phenomenon in which electrons are ejected from a metal surface when light shines on it. This effect played a crucial role in demonstrating that light is not just a wave, but also has particle-like properties, helping to establish the foundation of quantum mechanics.
Classical Prediction vs. Reality:
According to classical wave theory:
- Increasing the intensity (brightness) of light should increase the energy of ejected electrons.
- Any frequency of light, given enough time, should eventually eject electrons.
However, experiments showed:
- Electrons are only ejected if the light frequency is above a certain threshold, regardless of intensity.
- Below that frequency, no electrons are emitted — even if the light is very bright.
- Once the frequency is above the threshold, higher frequency light produces more energetic electrons, not brighter light.
Einstein’s Explanation:
In 1905, Albert Einstein proposed that light is made up of discrete packets of energy called photons. The energy of each photon is proportional to its frequency: E=hfE = hf
Where:
- EE = energy of the photon
- hh = Planck’s constant
- ff = frequency of light
If a photon’s energy is greater than the work function (the minimum energy needed to release an electron from the metal), it can eject an electron. Extra energy becomes the kinetic energy of the electron.
Quantum Leap:
This explanation showed that light behaves like a particle in certain interactions, introducing the idea that energy is quantized — a cornerstone of quantum theory. Einstein won the Nobel Prize in Physics in 1921 for this work, not for relativity.
The photoelectric effect remains one of the most direct proofs that light possesses both wave-like and particle-like properties, illustrating the concept of wave-particle duality.