Library / Foundations: Physics / FPH-37
Foundations: Physics
Photoelectric effect
Maximum kinetic energy of electrons ejected by light.
- Ref
- FPH-37
- Rev
- v1
- Field
- Foundations: Physics
- Result
- Kmax [eV]
Emission requires hf at least equal to the work function φ. All energy terms must use the same unit.
Each photon gives all its energy to one electron; whatever is left after paying the work function becomes the electron’s kinetic energy.
Calculator
- Threshold wavelength hc/φ288.3 nm
- Stopping potential0.6594 V
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Below the threshold frequency there is no emission however bright the light is; brightness only changes the number of electrons.
- The stopping potential in volts equals Kmax in eV.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (eV) | Local step | Change in result |
|---|---|---|---|---|---|---|
| λ Wavelength | 150 to 280 nm | 0.128 | 3.966 | 3.838 | +10% | −68.4 % |
| φ Work function | 2 to 5 eV | 0.05937 | 2.959 | 2.9 | +10% | −65.2 % |
Sweep of Wavelength
| Wavelength (nm) | Maximum kinetic energy (eV) |
|---|---|
| 150 | 3.966 |
| 166.3 | 3.158 |
| 182.5 | 2.494 |
| 198.8 | 1.938 |
| 215 | 1.467 |
| 231.3 | 1.061 |
| 247.5 | 0.7095 |
| 263.8 | 0.4008 |
| 280 | 0.128 |
Sweep one variable
Variables and how each one contributes
Maximum kinetic energy
What the equation gives you. Everything else feeds it.
Wavelength
Wavelength of the light. Shorter wavelengths give more kinetic energy.
Work function
Minimum energy to free an electron (zinc 4.3). Lowers the kinetic energy one-for-one.
Worked example with the values above
- Photon energy: 1240/250 = 4.959 eV.
- Subtract the work function 4.3 eV.
- Kmax = 0.6594 eV.
Assumptions and limits
- One photon frees one electron.
- Clean surface.
Common mistakes
- Mixing joules and electronvolts.
- Thinking brighter light raises the electron energy.
Statistics and uncertainty
Surface contamination changes the real work function by tenths of an eV, a big fraction of Kmax near threshold.
Where it is used
- Photodetectors and night-vision devices.
- Choosing materials for light sensors.
Maths you need
- Photon energy: E = hc/λ
- Electronvolt: 1.602×10⁻¹⁹ J