The Photoelectric Effect: The Experiment That Proved Light Comes in Packets

Shine light on a clean metal surface and, under the right conditions, electrons fly out of it. That simple effect, the photoelectric effect, looks unremarkable, but the details of how it happens couldn’t be explained by the physics of the 19th century. Albert Einstein’s explanation in 1905 showed that light itself comes in packets of energy, now called photons. It won him the Nobel Prize in Physics and helped launch quantum theory, the foundation of our modern understanding of atoms and chemistry.

The effect

When light (or other electromagnetic radiation) of high enough frequency falls on a metal, electrons are emitted from the surface. These are called photoelectrons.

The effect was first noticed by Heinrich Hertz in 1887, who found that sparks jumped more easily between metal electrodes when ultraviolet light shone on them. Over the following years, Philipp Lenard and others studied it carefully, and found a set of results that made no sense.

The puzzling observations

Physicists at the time thought of light purely as a wave, with energy spread continuously across the wavefront. A brighter light was simply a bigger wave with more energy. Based on this, they expected:

• Any frequency of light should eject electrons if it’s bright enough.
• Brighter light should give electrons more energy.
• With dim light, electrons would need time to “soak up” enough energy, so there would be a delay.

What experiments actually showed was very different:

1. Threshold frequency. Below a certain minimum frequency (the threshold frequency, f₀), no electrons are emitted, however bright the light. For zinc, red and visible light do nothing, but ultraviolet light works.
2. Energy depends on frequency, not intensity. Above the threshold, the maximum kinetic energy of the emitted electrons increases with the frequency of the light, but doesn’t change at all when the light is made brighter.
3. Intensity affects the number. Brighter light (above the threshold) produces more electrons per second, not faster ones.
4. No delay. Electrons are emitted instantly, even in very dim light, as long as the frequency is above the threshold.

Classical wave theory couldn’t explain any of this.

Einstein’s explanation (1905)

Building on Max Planck’s 1900 idea that energy is exchanged in discrete amounts, Einstein proposed that light itself consists of particles, or packets of energy, later called photons. Each photon has energy:

E = hf

where h is Planck’s constant (6.626 × 10⁻³⁴ J s) and f is the frequency.

His key insight: one photon interacts with one electron, giving it all of its energy at once.

• To escape from the metal, an electron needs a minimum amount of energy, called the work function, φ (phi). This depends on the metal: it’s the energy holding the least-tightly-bound electrons in the surface.
• If a photon’s energy is less than φ, the electron can’t escape. Since energy comes in single-photon packets, adding more photons (brighter light) doesn’t help; each one is still individually too weak. This explains the threshold frequency: hf₀ = φ.
• If a photon’s energy is more than φ, the electron escapes, and the extra energy becomes its kinetic energy:

hf = φ + KE_max

• Brighter light means more photons per second, so more electrons are emitted, but each has the same maximum energy. This explains observations 2 and 3.
• There’s no delay because the energy is delivered in a single photon, not gradually accumulated.

Every puzzle was solved by one simple, testable idea. For calculations with this equation, see photoelectric effect calculations.

An analogy

Imagine trying to knock coconuts out of a tree by throwing balls. If each ball is too light (low frequency), no number of balls, however many you throw (high intensity), will knock a coconut down. But a single heavy enough ball (high frequency) knocks one down immediately, and heavier balls send the coconut flying faster. Throwing more heavy balls knocks down more coconuts. It’s not perfect, but it captures why the energy of each individual photon matters.

Work functions of some metals

Metal	Approximate work function (eV)	Threshold wavelength (nm)
Caesium	2.1	about 590 (yellow-orange light)
Potassium	2.3	about 540
Sodium	2.3	about 540
Calcium	2.9	about 430
Zinc	4.3	about 290 (UV)
Copper	4.7	about 265 (UV)
Platinum	5.6	about 220 (UV)

(Values vary with surface condition and measurement.)

The alkali metals, especially caesium, have the lowest work functions, which fits their low ionisation energies: their outer electrons are held weakly. See alkali metals and ionization energy trend. That’s why caesium is used in light-sensitive devices that respond to visible light.

Millikan’s confirmation

Einstein’s idea was controversial. Many physicists, including Robert Millikan, doubted it. Millikan spent about ten years carrying out careful experiments, measuring the maximum kinetic energy of photoelectrons for different frequencies. He found a perfectly straight-line relationship between KE_max and f, with a gradient equal to Planck’s constant, just as Einstein had predicted. Einstein received the Nobel Prize in 1921 (awarded 1922) “especially for his discovery of the law of the photoelectric effect”, and Millikan received his in 1923, partly for this work.

Wave–particle duality

The photoelectric effect shows that light behaves as particles. But experiments such as diffraction and interference show that light also behaves as a wave. The resolution is wave–particle duality: light (and, as Louis de Broglie later proposed, electrons too) has both wave-like and particle-like properties, depending on how you observe it. This idea is central to quantum mechanics, which describes electrons in atoms. See what is an electron? and photons and energy levels.

Uses of the photoelectric effect

• Photomultiplier tubes detect extremely faint light by using the photoelectric effect and then multiplying the electron signal. They’re used in scientific instruments, including some spectrometers and medical scanners.
• Photoelectron spectroscopy fires photons of known energy at samples and measures the energies of the ejected electrons, revealing the energies of electrons in atoms and molecules. It’s direct evidence for shells and subshells.
• Night-vision devices amplify faint light using photoemission.
• Solar cells and camera sensors rely on a closely related process (the internal photoelectric effect), in which photons free electrons inside a semiconductor rather than ejecting them from its surface. See semiconductors explained.
• Automatic doors and light sensors historically used photoelectric cells.

It’s worth remembering how surprising this was in 1905: light had been firmly established as a wave for nearly a century.

Key takeaways

• The photoelectric effect is the emission of electrons from a metal when light of high enough frequency falls on it.
• Below a threshold frequency, no electrons are emitted, however bright the light; above it, electron energy depends on frequency and electron numbers on intensity.
• Einstein explained this in 1905: light consists of photons with E = hf, and one photon ejects one electron.
• hf = φ + KE_max, where φ is the metal’s work function.
• The effect proved light has particle properties and helped found quantum theory.