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Photoelectric Effect

Photoelectric Effect

10 min read Updated Mar 26, 2026

The photoelectric effect is one of the most important experiments in modern physics, and the MCAT tests it directly. It proved that light is not just a wave - it also behaves as particles (photons), each carrying a discrete packet of energy.

The Experiment

When light shines on a metal surface, electrons can be ejected from the metal. This is the photoelectric effect. But the results do not match what classical wave theory predicts:

Classical prediction: Brighter light (higher intensity) should give electrons more energy and eventually eject them, regardless of the light’s color (frequency).

Actual observation: Only light above a certain threshold frequency ejects electrons. Below this frequency, no electrons are ejected no matter how bright the light. Above the threshold, even dim light ejects electrons instantly.

Einstein’s Explanation

Einstein explained the photoelectric effect in 1905 by proposing that light consists of individual particles (photons), each with energy E = hf. His key insight:

  • Each photon interacts with one electron
  • If the photon’s energy (hf) is greater than or equal to the work function (phi) of the metal, the electron is ejected
  • If the photon’s energy is less than the work function, nothing happens - no matter how many photons hit the surface

Key Concepts

Threshold frequency (fβ‚€): The minimum frequency of light needed to eject electrons. At the threshold: hfβ‚€ = phi, so KE = 0 (the electron barely escapes).

Work function (phi): The minimum energy required to remove an electron from the metal surface. Different metals have different work functions. Metals with low work functions (like cesium) eject electrons more easily.

Above threshold: Any photon with f > fβ‚€ will eject an electron. The excess energy (hf - phi) becomes the kinetic energy of the ejected electron.

Intensity vs. Frequency

This distinction is the core of what the MCAT tests:

| Factor | What it affects | What it does NOT affect |
|--------|----------------|----------------------|
| Frequency (color) | Whether electrons are ejected; KE of each ejected electron | Number of electrons ejected |
| Intensity (brightness) | Number of electrons ejected (if above threshold) | Whether electrons are ejected; KE of ejected electrons |

  • Increasing frequency (above threshold): Each ejected electron has more kinetic energy
  • Increasing intensity (above threshold): More electrons are ejected per second, but each has the same KE
  • Increasing intensity (below threshold): Nothing happens. Zero electrons ejected. No exceptions.

Graphical Representation

The photoelectric effect: what frequency changes and what brightness changes

Quantised light
visible light 0 1 2 3 0.000.250.500.751.001.25 frequency (Γ—10¹⁡ Hz) max kinetic energy (eV) Each dot marks that metal's threshold frequency. Colours are keyed in the table. metal Ο† fβ‚€ = Ο†/h longest Ξ» that works Caesium 2.10 eV 5.08 Γ—10¹⁴ Hz 590 nm Β· visible Sodium 2.28 eV 5.51 Γ—10¹⁴ Hz 544 nm Β· visible Zinc 4.30 eV 10.40 Γ—10¹⁴ Hz 288 nm Β· ultraviolet Copper 4.70 eV 11.36 Γ—10¹⁴ Hz 264 nm Β· ultraviolet Raise the frequency Each photon carries more energy, so every electron leaves faster. The count is unchanged. three electrons, leaving fast Below fβ‚€ this panel is empty: no photon has enough energy, so nothing leaves. Raise the intensity More photons arrive, so more electrons come off. Each one leaves at exactly the same speed. six electrons, all leaving at the original speed KE(max) = hf βˆ’ Ο† frequency sets the energy Β· intensity sets the count
1

Scroll sideways to see the whole map.

Why every line is parallelThe slope of each line is Planck's constant. It does not depend on which metal you shine light at, because it is a fact about the photon, not the surface. Only the intercept moves.

What the intercept meansWhere a line crosses the axis is that metal's threshold frequency, fβ‚€ = Ο†/h. Below it the graph does not go negative, it simply stops: no electrons come off at all, and turning up the intensity does not change that.

The result that broke classical physicsA wave should let you accumulate energy, so a dim blue lamp ought to eventually free an electron and a bright red one certainly should. Neither happens. Energy arrives in single photons of E = hf, and one photon either carries enough to pay the work function or it does not.

Brightness and colour do completely different jobs. Raising the intensity sends more photons, so more electrons come off, but each one leaves with exactly the same kinetic energy. Raising the frequency raises the energy of every electron. That split is the whole experiment.

MCAT passages may show a graph of kinetic energy vs. frequency for the photoelectric effect:

  • The graph is a straight line with slope = h (Planck’s constant)
  • The x-intercept is the threshold frequency (fβ‚€)
  • The y-intercept (extrapolated) gives -phi (negative work function)
  • Below fβ‚€, the line does not exist - KE = 0, no electrons ejected
Light with frequency 8.0Γ—10148.0 \times 10^{14} Hz hits a metal with work function 3.0Γ—10βˆ’193.0 \times 10^{-19} J. Are electrons ejected? If so, what is their kinetic energy? (h=6.626Γ—10βˆ’34h = 6.626 \times 10^{-34} JΒ·s)
Click to reveal answer

Yes, electrons are ejected. KE = 2.3Γ—10βˆ’192.3 \times 10^{-19} J. Photon energy = hf=(6.626Γ—10βˆ’34)(8.0Γ—1014)=5.3Γ—10βˆ’19hf = (6.626 \times 10^{-34})(8.0 \times 10^{14}) = 5.3 \times 10^{-19} J. This exceeds the work function (3.0Γ—10βˆ’193.0 \times 10^{-19} J), so electrons are ejected. KE=hfβˆ’Ο•=5.3Γ—10βˆ’19βˆ’3.0Γ—10βˆ’19=2.3Γ—10βˆ’19KE = hf - \phi = 5.3 \times 10^{-19} - 3.0 \times 10^{-19} = 2.3 \times 10^{-19} J.

If you double the intensity of light that is below the threshold frequency, what happens?
Click to reveal answer

Absolutely nothing. No electrons are ejected. Doubling the intensity means doubling the number of photons, but each photon still lacks sufficient energy (hf < phi) to eject an electron. The photoelectric effect requires each individual photon to have enough energy - you cannot β€œadd up” the energies of multiple low-energy photons.

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