Light knocks electrons out of metals only if each photon has enough energy – no matter how bright the light. Einstein explained this in 1905 using light quanta, winning his Nobel Prize.
How to use the Photoelectric Effect Calculator
- Light given by
- Wavelength λ
- Frequency f
- Photon energy
- Metal
- Work function φ
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose how you describe the light and the metal (or a custom work function).
Inputs explained
| Field | What to enter |
|---|---|
| Light given by | Choose one: Wavelength, Frequency, Photon energy |
| Wavelength λ | Units: nm, µm, mm, m, pm, Å |
| Frequency f | Units: THz, PHz, Hz |
| Photon energy | Units: eV, keV, MeV, J, zJ |
| Metal | Choose one: Caesium, Potassium, Sodium, Calcium … |
| Work function φ | Units: eV, keV, MeV, J, zJ |
What each result means
- Maximum kinetic energy = [Photon energy] – [phi]
- Stopping potential = max(0, [Photon energy] – [phi])
- Threshold wavelength = 6.62607015e-34×299792458÷([phi]×1.602176634e-19)
Example values
How the main result changes when Wavelength λ changes and everything else stays at the example values above:
| Wavelength λ (nm) | Maximum kinetic energy (eV) |
|---|---|
| 125 | 5.2687 |
| 187.5 | 1.9625 |
| 250 | 0.3094 |
| 312.5 | -0.6825 |
| 375 | -1.3438 |
| 500 | -2.1703 |
Formula
K_max = h·f − φ, V_stop = K_max/e, λ_threshold = h·c/φ
Worked example
Inputs — Light given by: Wavelength; Wavelength λ: 250 nm; Metal: Copper.
Results — Photon energy = 4.959368 eV; Maximum kinetic energy = 0.309368 eV; Stopping potential = 0.309368 V; Threshold wavelength = 266.63268 nm; Electrons emitted? = Yes.
Key terms
- Work function
- Energy needed to remove an electron from a solid.
Frequently asked questions
Why doesn't brighter red light eject electrons?
Each photon still has too little energy.
What is the work function?
Minimum energy to free an electron from the surface.
What is the stopping potential?
The reverse voltage that just stops the fastest electrons.
Which metal has the lowest work function?
Caesium, about 2.1 eV.