Explaining Photoelectric Observations
Now change one property of the radiation at a time. The results separate the roles of frequency and intensity.
Change frequency and intensity separately
A fair comparison changes only one property of the incident radiation. When frequency is changed, keep intensity fixed. When intensity is changed, keep frequency and the metal fixed.
Each metal has a minimum frequency that can cause emission. Below this boundary, no photoelectrons are emitted. This boundary is called the threshold frequency; its exact definition and calculation are taught in the next lesson.
Test frequency and intensity
Change one control at a time. Read what changes and what remains constant.
Photon energy
4.56 eV
Minimum frequency
0.56 × 10¹⁵ Hz
Fastest electron KE
2.26 eV
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| Change | Observed result | What it tells you |
|---|---|---|
| Frequency below the threshold | No photoelectrons are emitted, even at high intensity. | No single photon has enough energy. |
| Frequency increased above the threshold | Maximum photoelectron kinetic energy increases. | Each photon carries more energy. |
| Intensity increased above the threshold | At fixed frequency, photoelectric current is directly proportional to intensity. | Intensity is proportional to photon rate, so electron emission rate increases in the same proportion. |
| Intensity increased above the threshold | Maximum kinetic energy does not change. | The energy of each photon is unchanged. |
| Very low intensity above the threshold | Emission still begins without a measurable delay. | One photon can transfer its energy to one electron immediately. |
Key idea
The photon model explains every observation
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One electron absorbs one complete photon. The photon disappears and transfers all its energy to that electron.
The electron must use a minimum amount of energy to escape the metal. In the A Level photon model, an electron does not save small amounts of energy from several photons. If one photon carries less than the minimum, increasing intensity still cannot cause emission.
Above the threshold, a larger intensity supplies more photons each second. More electrons can be released, but the energy carried by each photon is unchanged. The maximum kinetic energy is therefore unchanged.
Swipe left or right to read every column →
| Change | Changes | Does not change |
|---|---|---|
| Increase frequency above the threshold | photon energy and maximum photoelectron kinetic energy | work function of the metal |
| Increase intensity at fixed frequency | photon arrival rate, emission rate and photoelectric current | photon energy and maximum photoelectron kinetic energy |
| Change the metal | minimum escape energy and threshold frequency | Planck constant |
Why can radiation below the threshold frequency fail to emit electrons even when its intensity is high?
Show worked answer
An electron absorbs one whole photon. Each photon has energy below the minimum needed for escape, so no electron is emitted.
Explain photoelectric observations with the one-photon–one-electron interaction. Do not say that intensity increases photon energy.