Photoelectric Effect Calculator
Maximum kinetic energy and speed of the electrons knocked out of a metal by light, the stopping voltage, and the threshold wavelength and frequency.
Results
The dashed line marks the value you entered.
What this tool does
Einstein explained the photoelectric effect in 1905, and it is what won him the Nobel Prize: light arrives in packets of energy hc/λ, and an electron escapes only if one packet beats the metal's work function. That is why brighter red light frees no electrons from zinc while faint ultraviolet does; brightness changes how many electrons come out, never their top energy. Millikan used the stopping voltage, the reverse voltage that just halts the fastest electrons, to measure Planck's constant in 1916. Work functions are roughly 2.3 eV for sodium and potassium and 4.3 eV for zinc, and they shift with surface cleanliness and oxidation.
Formula
E = hc ÷ λ · Kmax = E − φ · V₀ = Kmax ÷ e · λ₀ = hc ÷ φ
Variables
| Symbol | Meaning | Unit |
|---|---|---|
lam | Wavelength | nm |
wf | Work function of the metal | eV |
KE | Maximum kinetic energy of the electrons | eV |
EP | Energy of each photon | eV |
VS | Stopping voltage | V |
LT | Longest wavelength that still frees electrons | nm |
FT | Threshold frequency | THz |
VM | Top speed of the electrons | km/s |
Worked example
- Wavelength400 nm
- Work function of the metal2.3 eV
- Maximum kinetic energy of the electrons0.7996 eV
- Energy of each photon3.0996 eV
- Stopping voltage0.7996 V
- Longest wavelength that still frees electrons539.1 nm
- Threshold frequency556.1 THz
- Top speed of the electrons530 km/s
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.