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The Photoelectric Effect Einstein’s 1905 Explanation Earned the Nobel Prize. In addition to his work on relativity in 1905, Einstein published an explanation for the photoelectric effect that earned him the 1921 Nobel Prize.

The Photoelectric Effect

Solar powered calculators and solar power cells work on the photoelectric effect. When light strikes certain conducting materials, the electrons absorb enough energy to escape the atoms. These electrons travel through the conducting materials and set up an electric current.

Photoelectric cells therefore convert light into electrical energy.

The Problem with the Photoelectric Effect

Early experimental studies of the photoelectric effect produced results that 19th century physics could not explain.

For a given photoelectric material, light with wavelengths shorter than a specific threshold wavelength (or a frequency higher than a threshold frequency) produced the photoelectric effect. A low intensity of light only released a few electrons. A brighter light released more electrons. Physicists understood this part. More intense, brighter, light has more total energy to release more electrons.

Physicists did not however understand why the wavelength had to be shorter than a threshold wavelength. Ultraviolet light could cause the photoelectric effect even at very low intensity. While longer wavelength red or infrared light could not free the electrons no matter how intense the light. Thinking that a sufficiently bright red light, with more total energy than a faint blue light, should have enough energy to trigger the photoelectric effect, physicists of the day were puzzled.

Einstein’s Solution

In 1905 Einstein explained the photoelectric effect. Einstein was working as a clerk in the Swiss patent office during this miracle year, in which he also published his special theory of relativity and his explanation for Brownian motion showing atoms exist.

Einstein’s realized that light is quantized. It comes in very small discrete chunks or quanta called photons. A photon of light is essentially the smallest possible amount of light.

A photon of red or infrared light has less energy than a photon of blue or ultraviolet light. To free an electron from its atom, a photon of light strikes the electron. If the photon has enough energy, the electron escapes the atom, and we observe the photoelectric effect. If the photon is a lower energy photon, it lacks the energy to free the electron. We do not observe the photoelectric effect, no matter how intense the light.

The electron gets the energy from one photon and longer wavelength photons have insufficient energy per photon. Shining more photons from an intense light does not increase the energy per photon.

Sowing Seeds of Quantum Mechanics

One of the fundamental principles of quantum physics is that quantities are quantized. They come in discrete chunks or quanta. We can have 1, 2, or any integer number of photons of light, but we can not have a fractional number (like 1.3) of photons.

The same is true for all other quantities. However the quanta (or the discrete chunks) are so small that we can only observe quantum effects on the smallest microscopic scales. The discrete quantum values are so close together that the quantum jumps are too small to measure in macroscopic objects. Quantum effects therefore apply to electrons and protons not to baseballs and basketballs.

By realizing that light is quantized Einstein paved the way for the quantum mechanical principle that quantities are quantized. By showing light had particle as well as wave properties, Einstein paved the way for Louis de Broglie’s idea of wave particle duality: Light and all fundamental particles have both particle and wave properties.

Einstein’s explanation of the photoelectric effect sowed the seeds for quantum mechanics, but Einstein himself never really thought quantum mechanics was correct. Quantum mechanics is also based on chance and Einstein did not think that God would choose to play dice with the universe.





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